diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4489304..b77eca8 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -13,10 +13,11 @@ jobs:
         php:
           - '8.0'
           - '8.1'
+          - '8.2'
         coverage:
           - none
         include:
-          - php: '8.2'
+          - php: '8.3'
             coverage: xdebug
     steps:
       - uses: shivammathur/setup-php@2.26.0
@@ -28,7 +29,7 @@ jobs:
           fetch-depth: 2 # required by Scrutinizer
       - run: composer install --no-progress --no-ansi --no-interaction --dev --prefer-dist
       - run: php test/test.php
-      - if: github.repository == 'mindplay-dk/unbox' && matrix.coverage == 'xdebug'
+      - if: matrix.coverage == 'xdebug'
         uses: sudo-bot/action-scrutinizer@latest
         with:
           cli-args: '--format=php-clover test/build/clover.xml'
diff --git a/.gitignore b/.gitignore
index 9bab9ec..613b203 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
 vendor
 .*
 !.git*
+/composer.lock
diff --git a/README.md b/README.md
index cf70351..aefcf6c 100644
--- a/README.md
+++ b/README.md
@@ -167,6 +167,9 @@ ref(string $name) : BoxedValueInterface                # create a boxed referenc
 
 registerFallback(ContainerInterface $container)        # register a fallack container
 
+requires(string $requirement, string $description)     # defines a Requirement
+provides(string $requirement, string $description)     # fulfills an abstract Requirement
+
 createContainer() : Container                          # create a bootstrapped Container instance
 ```
 
@@ -524,6 +527,82 @@ for a quick development setup. Even if somebody wanted to override some of the r
 in e.g. your default development setup, they can of course still do that, e.g. by calling
 `register()` again to override components as needed.
 
+##### Provider Requirements
+
+In large, modular architectures, you may have many Providers with inter-dependencies, which
+can become difficult to manage at scale.
+
+Since Providers exist *outside* the realm of the Container, the concept of Requirements can
+be used to define verifiable provider interdependencies, which will be checked at the time
+when `createContainer()` is called.
+
+Requirements may be defined by calling `requires()`, and more than one Provider may specify the
+same Requirement - possibly for different reasons, which may be described using the optional
+`$description` argument.
+
+Requirements may be fulfilled by calling either `register()` or `provides()`.
+
+###### Component Requirements
+
+Providers may depend on the consumer to manually register a component.
+ 
+For example, the following Provider requires you to register a `PDO` connection instance:
+
+```php
+class MyProvider implements ProviderInterface
+{
+    public function register(ContainerFactory $factory)
+    {
+        $factory->requires(PDO::class, "a PDO instance connected to a MySQL database");
+        
+        // ...
+    }
+}
+```
+
+Attempting to bootstrap this Provider, without manually registering the `PDO` instance, will
+generate an Exception, as soon as `createContainer()` is called - which is much easier to debug
+than the `NotFoundException` you would otherwise get, and which might not occur until you try
+to resolve a component that actually depends on the database connection.
+
+###### Abstract Requirements
+
+Providers may have abstract Requirements - something that can't be expressed by a simple
+component dependency.
+
+For example, the following Provider requires you to simply indicate that you've bootstrapped
+a "payment gateway" - whatever that means to the Provider in question:
+
+```php
+class MyProvider implements ProviderInterface
+{
+    public function register(ContainerFactory $factory)
+    {
+        $factory->requires("acme.payment-gateway", "please refer to acme's documentation");
+        
+        // ...
+    }
+}
+```
+
+Another provider needs to explicitly indicate fulfillment of this abstract Requirement:
+
+```php
+class MyPaymentProvider implements ProviderInterface
+{
+    public function register(ContainerFactory $factory)
+    {
+        $factory->provides("acme.payment-gateway");
+        
+        // ...
+    }
+}
+```
+
+Note that abstract Requirements should be a last resort - component dependencies are
+*generally* simpler and easier to understand. This feature exists primarily to support
+complex, large-scale modular frameworks.
+
 ### Fallback Containers
 
 You can use this feature to build layered architecture with different component life-cycles.
diff --git a/UPGRADING.md b/UPGRADING.md
index cb4ce90..371d194 100644
--- a/UPGRADING.md
+++ b/UPGRADING.md
@@ -1,6 +1,10 @@
 Upgrading
 =========
 
+#### 3.1.0
+
+This feature-release introduces requirements (via `requires` and `provides`) and requires PHP 8.0 or later.
+
 #### 3.0.0
 
 This release removes backwards compatibility with the legacy package `container-interop/container-interop`
diff --git a/composer.json b/composer.json
index ef405b0..9d9ec8d 100644
--- a/composer.json
+++ b/composer.json
@@ -15,10 +15,13 @@
     },
     "require-dev": {
         "mindplay/benchpress": "^0.1",
-        "mindplay/testies": "^1.0",
+        "mindplay/testies": "^1.1.2",
         "php-di/php-di": "^7.0.5",
         "pimple/pimple": "^3.5",
-        "phpunit/php-code-coverage": "^9.2.5"
+        "phpunit/php-code-coverage": "^9 || ^10 || ^11"
+    },
+    "scripts": {
+        "test": "XDEBUG_MODE=coverage php test/test.php"
     },
     "autoload": {
         "psr-4": {
diff --git a/composer.lock b/composer.lock
deleted file mode 100644
index 29d0a04..0000000
--- a/composer.lock
+++ /dev/null
@@ -1,994 +0,0 @@
-{
-    "_readme": [
-        "This file locks the dependencies of your project to a known state",
-        "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
-        "This file is @generated automatically"
-    ],
-    "content-hash": "c8c86b3335bd2bb7d08db2bfe29f8175",
-    "packages": [
-        {
-            "name": "psr/container",
-            "version": "2.0.2",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/php-fig/container.git",
-                "reference": "c71ecc56dfe541dbd90c5360474fbc405f8d5963"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/php-fig/container/zipball/c71ecc56dfe541dbd90c5360474fbc405f8d5963",
-                "reference": "c71ecc56dfe541dbd90c5360474fbc405f8d5963",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=7.4.0"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "2.0.x-dev"
-                }
-            },
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-                }
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "MIT"
-            ],
-            "authors": [
-                {
-                    "name": "PHP-FIG",
-                    "homepage": "https://www.php-fig.org/"
-                }
-            ],
-            "description": "Common Container Interface (PHP FIG PSR-11)",
-            "homepage": "https://github.com/php-fig/container",
-            "keywords": [
-                "PSR-11",
-                "container",
-                "container-interface",
-                "container-interop",
-                "psr"
-            ],
-            "support": {
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-                "source": "https://github.com/php-fig/container/tree/2.0.2"
-            },
-            "time": "2021-11-05T16:47:00+00:00"
-        }
-    ],
-    "packages-dev": [
-        {
-            "name": "laravel/serializable-closure",
-            "version": "v1.3.1",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/laravel/serializable-closure.git",
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-            },
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-                "shasum": ""
-            },
-            "require": {
-                "php": "^7.3|^8.0"
-            },
-            "require-dev": {
-                "nesbot/carbon": "^2.61",
-                "pestphp/pest": "^1.21.3",
-                "phpstan/phpstan": "^1.8.2",
-                "symfony/var-dumper": "^5.4.11"
-            },
-            "type": "library",
-            "extra": {
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-                }
-            },
-            "autoload": {
-                "psr-4": {
-                    "Laravel\\SerializableClosure\\": "src/"
-                }
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "MIT"
-            ],
-            "authors": [
-                {
-                    "name": "Taylor Otwell",
-                    "email": "taylor@laravel.com"
-                },
-                {
-                    "name": "Nuno Maduro",
-                    "email": "nuno@laravel.com"
-                }
-            ],
-            "description": "Laravel Serializable Closure provides an easy and secure way to serialize closures in PHP.",
-            "keywords": [
-                "closure",
-                "laravel",
-                "serializable"
-            ],
-            "support": {
-                "issues": "https://github.com/laravel/serializable-closure/issues",
-                "source": "https://github.com/laravel/serializable-closure"
-            },
-            "time": "2023-07-14T13:56:28+00:00"
-        },
-        {
-            "name": "mindplay/benchpress",
-            "version": "0.1.0",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/mindplay-dk/benchpress.git",
-                "reference": "e46935bb0c220ba2ecd6d32314f875f3d8511f16"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/mindplay-dk/benchpress/zipball/e46935bb0c220ba2ecd6d32314f875f3d8511f16",
-                "reference": "e46935bb0c220ba2ecd6d32314f875f3d8511f16",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.3.0"
-            },
-            "type": "library",
-            "autoload": {
-                "psr-4": {
-                    "mindplay\\benchpress\\": "src/"
-                }
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "LGPL-3.0+"
-            ],
-            "authors": [
-                {
-                    "name": "Rasmus Schultz",
-                    "email": "rasmus@mindplay.dk"
-                }
-            ],
-            "description": "A simple benchmark suite for PHP 5.3 and up",
-            "support": {
-                "issues": "https://github.com/mindplay-dk/benchpress/issues",
-                "source": "https://github.com/mindplay-dk/benchpress/tree/master"
-            },
-            "time": "2016-08-07T13:44:25+00:00"
-        },
-        {
-            "name": "mindplay/testies",
-            "version": "1.0.0",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/mindplay-dk/testies.git",
-                "reference": "6501bc0ec4599948ee83284aa926538af839de50"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/mindplay-dk/testies/zipball/6501bc0ec4599948ee83284aa926538af839de50",
-                "reference": "6501bc0ec4599948ee83284aa926538af839de50",
-                "shasum": ""
-            },
-            "require": {
-                "php": "^7.3 || ^8.0"
-            },
-            "require-dev": {
-                "guzzlehttp/guzzle": "^6.3.3",
-                "phpunit/php-code-coverage": "^9.2.5"
-            },
-            "suggest": {
-                "phpunit/php-code-coverage": "^9.2.5"
-            },
-            "type": "library",
-            "autoload": {
-                "files": [
-                    "src/test.func.php"
-                ],
-                "psr-4": {
-                    "mindplay\\testies\\": "src/"
-                }
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "MIT"
-            ],
-            "authors": [
-                {
-                    "name": "Rasmus Schultz",
-                    "email": "rasmus@mindplay.dk"
-                }
-            ],
-            "description": "Yeah, testies: a lightweight library for quick, simple unit-testing",
-            "support": {
-                "issues": "https://github.com/mindplay-dk/testies/issues",
-                "source": "https://github.com/mindplay-dk/testies/tree/1.0.0"
-            },
-            "time": "2021-02-25T09:50:25+00:00"
-        },
-        {
-            "name": "nikic/php-parser",
-            "version": "v4.17.1",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/nikic/PHP-Parser.git",
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-            },
-            "require": {
-                "ext-tokenizer": "*",
-                "php": ">=7.0"
-            },
-            "require-dev": {
-                "ircmaxell/php-yacc": "^0.0.7",
-                "phpunit/phpunit": "^6.5 || ^7.0 || ^8.0 || ^9.0"
-            },
-            "bin": [
-                "bin/php-parse"
-            ],
-            "type": "library",
-            "extra": {
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-                }
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-            ],
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-                "php"
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-            "time": "2023-08-13T19:53:39+00:00"
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-            "version": "2.3.4",
-            "source": {
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-                "url": "https://github.com/PHP-DI/Invoker.git",
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-                "MIT"
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-                "php": ">=8.0",
-                "php-di/invoker": "^2.0",
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-            },
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-            },
-            "suggest": {
-                "friendsofphp/proxy-manager-lts": "Install it if you want to use lazy injection (version ^1)"
-            },
-            "type": "library",
-            "autoload": {
-                "files": [
-                    "src/functions.php"
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-            "description": "The dependency injection container for humans",
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-                "container",
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-            "time": "2023-08-10T14:57:56+00:00"
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-        },
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-            "require": {
-                "php": ">=7.3"
-            },
-            "require-dev": {
-                "phpunit/phpunit": "^9.3"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "3.0-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de",
-                    "role": "lead"
-                }
-            ],
-            "description": "FilterIterator implementation that filters files based on a list of suffixes.",
-            "homepage": "https://github.com/sebastianbergmann/php-file-iterator/",
-            "keywords": [
-                "filesystem",
-                "iterator"
-            ],
-            "support": {
-                "issues": "https://github.com/sebastianbergmann/php-file-iterator/issues",
-                "source": "https://github.com/sebastianbergmann/php-file-iterator/tree/3.0.6"
-            },
-            "funding": [
-                {
-                    "url": "https://github.com/sebastianbergmann",
-                    "type": "github"
-                }
-            ],
-            "time": "2021-12-02T12:48:52+00:00"
-        },
-        {
-            "name": "phpunit/php-text-template",
-            "version": "2.0.4",
-            "source": {
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-                "url": "https://github.com/sebastianbergmann/php-text-template.git",
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-                    "email": "sebastian@phpunit.de",
-                    "role": "lead"
-                }
-            ],
-            "description": "Simple template engine.",
-            "homepage": "https://github.com/sebastianbergmann/php-text-template/",
-            "keywords": [
-                "template"
-            ],
-            "support": {
-                "issues": "https://github.com/sebastianbergmann/php-text-template/issues",
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-                    "type": "github"
-                }
-            ],
-            "time": "2020-10-26T05:33:50+00:00"
-        },
-        {
-            "name": "pimple/pimple",
-            "version": "v3.5.0",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/silexphp/Pimple.git",
-                "reference": "a94b3a4db7fb774b3d78dad2315ddc07629e1bed"
-            },
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-                "reference": "a94b3a4db7fb774b3d78dad2315ddc07629e1bed",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=7.2.5",
-                "psr/container": "^1.1 || ^2.0"
-            },
-            "require-dev": {
-                "symfony/phpunit-bridge": "^5.4@dev"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "3.4.x-dev"
-                }
-            },
-            "autoload": {
-                "psr-0": {
-                    "Pimple": "src/"
-                }
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "MIT"
-            ],
-            "authors": [
-                {
-                    "name": "Fabien Potencier",
-                    "email": "fabien@symfony.com"
-                }
-            ],
-            "description": "Pimple, a simple Dependency Injection Container",
-            "homepage": "https://pimple.symfony.com",
-            "keywords": [
-                "container",
-                "dependency injection"
-            ],
-            "support": {
-                "source": "https://github.com/silexphp/Pimple/tree/v3.5.0"
-            },
-            "time": "2021-10-28T11:13:42+00:00"
-        },
-        {
-            "name": "sebastian/code-unit-reverse-lookup",
-            "version": "2.0.3",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/code-unit-reverse-lookup.git",
-                "reference": "ac91f01ccec49fb77bdc6fd1e548bc70f7faa3e5"
-            },
-            "dist": {
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-                "url": "https://api.github.com/repos/sebastianbergmann/code-unit-reverse-lookup/zipball/ac91f01ccec49fb77bdc6fd1e548bc70f7faa3e5",
-                "reference": "ac91f01ccec49fb77bdc6fd1e548bc70f7faa3e5",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=7.3"
-            },
-            "require-dev": {
-                "phpunit/phpunit": "^9.3"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "2.0-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de"
-                }
-            ],
-            "description": "Looks up which function or method a line of code belongs to",
-            "homepage": "https://github.com/sebastianbergmann/code-unit-reverse-lookup/",
-            "support": {
-                "issues": "https://github.com/sebastianbergmann/code-unit-reverse-lookup/issues",
-                "source": "https://github.com/sebastianbergmann/code-unit-reverse-lookup/tree/2.0.3"
-            },
-            "funding": [
-                {
-                    "url": "https://github.com/sebastianbergmann",
-                    "type": "github"
-                }
-            ],
-            "time": "2020-09-28T05:30:19+00:00"
-        },
-        {
-            "name": "sebastian/complexity",
-            "version": "2.0.2",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/complexity.git",
-                "reference": "739b35e53379900cc9ac327b2147867b8b6efd88"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/complexity/zipball/739b35e53379900cc9ac327b2147867b8b6efd88",
-                "reference": "739b35e53379900cc9ac327b2147867b8b6efd88",
-                "shasum": ""
-            },
-            "require": {
-                "nikic/php-parser": "^4.7",
-                "php": ">=7.3"
-            },
-            "require-dev": {
-                "phpunit/phpunit": "^9.3"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "2.0-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de",
-                    "role": "lead"
-                }
-            ],
-            "description": "Library for calculating the complexity of PHP code units",
-            "homepage": "https://github.com/sebastianbergmann/complexity",
-            "support": {
-                "issues": "https://github.com/sebastianbergmann/complexity/issues",
-                "source": "https://github.com/sebastianbergmann/complexity/tree/2.0.2"
-            },
-            "funding": [
-                {
-                    "url": "https://github.com/sebastianbergmann",
-                    "type": "github"
-                }
-            ],
-            "time": "2020-10-26T15:52:27+00:00"
-        },
-        {
-            "name": "sebastian/environment",
-            "version": "5.1.5",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/environment.git",
-                "reference": "830c43a844f1f8d5b7a1f6d6076b784454d8b7ed"
-            },
-            "dist": {
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-                "url": "https://api.github.com/repos/sebastianbergmann/environment/zipball/830c43a844f1f8d5b7a1f6d6076b784454d8b7ed",
-                "reference": "830c43a844f1f8d5b7a1f6d6076b784454d8b7ed",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=7.3"
-            },
-            "require-dev": {
-                "phpunit/phpunit": "^9.3"
-            },
-            "suggest": {
-                "ext-posix": "*"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "5.1-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de"
-                }
-            ],
-            "description": "Provides functionality to handle HHVM/PHP environments",
-            "homepage": "http://www.github.com/sebastianbergmann/environment",
-            "keywords": [
-                "Xdebug",
-                "environment",
-                "hhvm"
-            ],
-            "support": {
-                "issues": "https://github.com/sebastianbergmann/environment/issues",
-                "source": "https://github.com/sebastianbergmann/environment/tree/5.1.5"
-            },
-            "funding": [
-                {
-                    "url": "https://github.com/sebastianbergmann",
-                    "type": "github"
-                }
-            ],
-            "time": "2023-02-03T06:03:51+00:00"
-        },
-        {
-            "name": "sebastian/lines-of-code",
-            "version": "1.0.3",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/lines-of-code.git",
-                "reference": "c1c2e997aa3146983ed888ad08b15470a2e22ecc"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/lines-of-code/zipball/c1c2e997aa3146983ed888ad08b15470a2e22ecc",
-                "reference": "c1c2e997aa3146983ed888ad08b15470a2e22ecc",
-                "shasum": ""
-            },
-            "require": {
-                "nikic/php-parser": "^4.6",
-                "php": ">=7.3"
-            },
-            "require-dev": {
-                "phpunit/phpunit": "^9.3"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "1.0-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de",
-                    "role": "lead"
-                }
-            ],
-            "description": "Library for counting the lines of code in PHP source code",
-            "homepage": "https://github.com/sebastianbergmann/lines-of-code",
-            "support": {
-                "issues": "https://github.com/sebastianbergmann/lines-of-code/issues",
-                "source": "https://github.com/sebastianbergmann/lines-of-code/tree/1.0.3"
-            },
-            "funding": [
-                {
-                    "url": "https://github.com/sebastianbergmann",
-                    "type": "github"
-                }
-            ],
-            "time": "2020-11-28T06:42:11+00:00"
-        },
-        {
-            "name": "sebastian/version",
-            "version": "3.0.2",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/version.git",
-                "reference": "c6c1022351a901512170118436c764e473f6de8c"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/version/zipball/c6c1022351a901512170118436c764e473f6de8c",
-                "reference": "c6c1022351a901512170118436c764e473f6de8c",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=7.3"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "3.0-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de",
-                    "role": "lead"
-                }
-            ],
-            "description": "Library that helps with managing the version number of Git-hosted PHP projects",
-            "homepage": "https://github.com/sebastianbergmann/version",
-            "support": {
-                "issues": "https://github.com/sebastianbergmann/version/issues",
-                "source": "https://github.com/sebastianbergmann/version/tree/3.0.2"
-            },
-            "funding": [
-                {
-                    "url": "https://github.com/sebastianbergmann",
-                    "type": "github"
-                }
-            ],
-            "time": "2020-09-28T06:39:44+00:00"
-        },
-        {
-            "name": "theseer/tokenizer",
-            "version": "1.2.1",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/theseer/tokenizer.git",
-                "reference": "34a41e998c2183e22995f158c581e7b5e755ab9e"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/theseer/tokenizer/zipball/34a41e998c2183e22995f158c581e7b5e755ab9e",
-                "reference": "34a41e998c2183e22995f158c581e7b5e755ab9e",
-                "shasum": ""
-            },
-            "require": {
-                "ext-dom": "*",
-                "ext-tokenizer": "*",
-                "ext-xmlwriter": "*",
-                "php": "^7.2 || ^8.0"
-            },
-            "type": "library",
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Arne Blankerts",
-                    "email": "arne@blankerts.de",
-                    "role": "Developer"
-                }
-            ],
-            "description": "A small library for converting tokenized PHP source code into XML and potentially other formats",
-            "support": {
-                "issues": "https://github.com/theseer/tokenizer/issues",
-                "source": "https://github.com/theseer/tokenizer/tree/1.2.1"
-            },
-            "funding": [
-                {
-                    "url": "https://github.com/theseer",
-                    "type": "github"
-                }
-            ],
-            "time": "2021-07-28T10:34:58+00:00"
-        }
-    ],
-    "aliases": [],
-    "minimum-stability": "stable",
-    "stability-flags": [],
-    "prefer-stable": false,
-    "prefer-lowest": false,
-    "platform": {
-        "php": "^8.0"
-    },
-    "platform-dev": [],
-    "plugin-api-version": "2.6.0"
-}
diff --git a/src/Configuration.php b/src/Configuration.php
index 5d67333..5432277 100644
--- a/src/Configuration.php
+++ b/src/Configuration.php
@@ -37,6 +37,18 @@ abstract class Configuration
      */
     protected $fallbacks = [];
 
+    /**
+     * Check for the existence of a component with a given name.
+     *
+     * @param string $name component name
+     *
+     * @return bool true, if a component with the given name has been defined
+     */
+    public function has(string $name): bool
+    {
+        return array_key_exists($name, $this->values) || isset($this->factory[$name]);
+    }
+
     /**
      * Internally copy configuration state, e.g. from `ContainerFactory` to `Container`
      *
diff --git a/src/ContainerFactory.php b/src/ContainerFactory.php
index 0ea7a91..01820e6 100644
--- a/src/ContainerFactory.php
+++ b/src/ContainerFactory.php
@@ -11,6 +11,16 @@
  */
 class ContainerFactory extends Configuration
 {
+    /**
+     * @var string[][] map where Requirement ID => list of requirement descriptions
+     */
+    protected $required = [];
+
+    /**
+     * @var string[] map where Requirement ID => description of provided abstract requirements
+     */
+    protected $provided = [];
+
     public function __construct()
     {}
 
@@ -62,7 +72,7 @@ public function __construct()
      *
      * @throws InvalidArgumentException
      */
-    public function register($name, $func_or_map_or_type = null, $map = []): void
+    public function register(string $name, $func_or_map_or_type = null, $map = []): void
     {
         if (is_callable($func_or_map_or_type)) {
             // second argument is a creation function
@@ -241,19 +251,63 @@ public function ref(string $name): BoxedReference
     }
 
     /**
-     * Add a packaged configuration (a "provider") to this container.
-     *
-     * @see ProviderInterface
+     * Add a Provider (a packaged unit of bootstrapping) to this Container.
      *
      * @param ProviderInterface $provider
      *
      * @return void
+     *
+     * @see ProviderInterface
      */
     public function add(ProviderInterface $provider): void
     {
         $provider->register($this);
     }
 
+    /**
+     * Defines a Requirement, which will be checked when you call {@see createContainer()}.
+     *
+     * Requirements must be fulfilled by either {@see register()} or {@see provides()}.
+     *
+     * @param string $requirement Requirement name.
+     * @param string $description Optional description.
+     *                            Displayed in an Exception message on failure.
+     *
+     * @see provides()
+     */
+    public function requires(string $requirement, string $description = "")
+    {
+        $this->required[$requirement][] = $description;
+    }
+
+    /**
+     * Fulfills an abstract Requirement defined by {@see requires()}.
+     *
+     * @param string $requirement Requirement name.
+     * @param string $description Optional description.
+     *                            Displayed in an Exception message on failure.
+     *
+     * @throws ContainerException if the given Requirement has already been fulfilled
+     *
+     * @see requires()
+     */
+    public function provides(string $requirement, string $description = "")
+    {
+        if (array_key_exists($requirement, $this->provided)) {
+            $message = "The following Requirement has already been fulfilled: {$requirement}";
+
+            $description = $this->provided[$requirement];
+
+            if ($description) {
+                $message .= " ($description)";
+            }
+
+            throw new ContainerException($message);
+        }
+
+        $this->provided[$requirement] = $description;
+    }
+
     /**
      * Add a fallback container to this container.
      * 
@@ -281,9 +335,24 @@ public function registerFallback(ContainerInterface $container): void
      * Create and bootstrap a new `Container` instance
      *
      * @return Container
+     *
+     * @throws ContainerException if any Requirements have not been fulfilled
      */
     public function createContainer()
     {
+        $messages = [];
+
+        foreach ($this->required as $requirement => $descriptions) {
+            if (! array_key_exists($requirement, $this->provided) && !$this->has($requirement)) {
+                $messages[] = "The following Requirement has not been fulfilled: {$requirement}"
+                    . (array_filter($descriptions) ? " (" . implode("; ", $descriptions) . ")" : "");
+            }
+        }
+
+        if ($messages) {
+            throw new ContainerException(implode("\n", $messages));
+        }
+
         return new Container($this);
     }
 }
diff --git a/src/ProviderInterface.php b/src/ProviderInterface.php
index 5b92313..526d138 100644
--- a/src/ProviderInterface.php
+++ b/src/ProviderInterface.php
@@ -12,9 +12,9 @@ interface ProviderInterface
     /**
      * Registers services and components with a given `ContainerFactory`
      *
-     * @param ContainerFactory $container
+     * @param ContainerFactory $factory
      *
      * @return void
      */
-    public function register(ContainerFactory $container);
+    public function register(ContainerFactory $factory);
 }
diff --git a/test/fixtures.php b/test/fixtures.php
index 6285d8c..3fa4d07 100644
--- a/test/fixtures.php
+++ b/test/fixtures.php
@@ -96,15 +96,15 @@ public function __construct(UserRepository $users)
 
 class TestProvider implements ProviderInterface
 {
-    public function register(ContainerFactory $container)
+    public function register(ContainerFactory $factory)
     {
-        $container->set('cache_path', '/tmp/cache');
+        $factory->set('cache_path', '/tmp/cache');
 
-        $container->register(CacheProvider::class, function ($cache_path) {
+        $factory->register(CacheProvider::class, function ($cache_path) {
             return new FileCache($cache_path);
         });
 
-        $container->register(UserRepository::class, function (CacheProvider $cache) {
+        $factory->register(UserRepository::class, function (CacheProvider $cache) {
             return new UserRepository($cache);
         });
     }
diff --git a/test/test.php b/test/test.php
index 854d73e..3d71535 100644
--- a/test/test.php
+++ b/test/test.php
@@ -609,6 +609,107 @@ function (CacheProvider $provider) {
     }
 );
 
+test(
+    'can check abstract requirements',
+    function () {
+        $factory = new ContainerFactory();
+
+        $factory->requires("duck", "it really needs a duck");
+
+        // The same Requirement may be defined more than once:
+        $factory->requires("duck", "it really, really needs a duck");
+
+        expect(
+            ContainerException::class,
+            "should throw for missing requirement",
+            function () use ($factory) {
+                $factory->createContainer();
+            },
+            [
+                "/requirement has not been fulfilled/i",
+                "/duck \(it really needs a duck; it really, really needs a duck\)/i",
+            ]
+        );
+
+        $factory->provides("duck", "add that missing duck");
+
+        ok($factory->createContainer() instanceof Container);
+
+        expect(
+            ContainerException::class,
+            "should throw on attempt to provide a requirement twice",
+            function () use ($factory) {
+                $factory->provides("duck", "add another duck");
+            },
+            ["/requirement has already been fulfilled/i", "/duck \(add that missing duck\)/i"]
+        );
+    }
+);
+
+test(
+    'can check component requirements',
+    function () {
+        $factory = new ContainerFactory();
+
+        // component defined using register() in TestProvider
+        $factory->requires(CacheProvider::class, "requires a cache");
+
+        // component defined using set() in TestProvider
+        $factory->requires("cache_path", "requires a cache path");
+
+        expect(
+            ContainerException::class,
+            "should throw for missing requirement",
+            function () use ($factory) {
+                $factory->createContainer();
+            },
+            [
+                "/requirement has not been fulfilled/i",
+                "/CacheProvider \(requires a cache\)/i",
+                "/cache_path \(requires a cache path\)/i"
+            ]
+        );
+
+        $factory->add(new TestProvider());
+
+        ok($factory->createContainer() instanceof Container);
+    }
+);
+
+test(
+    'abstract requirements cannot fulfill component dependencies',
+    function () {
+        $factory = new ContainerFactory();
+
+        $factory->requires("cake", "I want cake!");
+
+        expect(
+            ContainerException::class,
+            "should throw for missing requirement",
+            function () use ($factory) {
+                $factory->createContainer();
+            },
+            "/requirement has not been fulfilled/i"
+        );
+
+        $factory->provides("cake", "I can haz cake.");
+
+        $container = $factory->createContainer();
+
+        expect(
+            NotFoundException::class,
+            "should throw for missing dependency",
+            function () use ($container) {
+                $container->get("cake");
+            }
+        );
+
+        $factory->set("cake", "yum");
+
+        eq($factory->createContainer()->get("cake"), "yum", "does not conflict with component dependencies");
+    }
+);
+
 test(
     'internal reflection guard clauses',
     function () {