diff --git a/.circleci/config.yml b/.circleci/config.yml
index 193ec51a9..066701cb4 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -42,14 +42,21 @@ jobs:
             - node_modules
           key: v2-dependencies-npm-{{ checksum "package-lock.json" }}
       - run: npm run uswds-build
-      - run: bundle exec jekyll build
+      # We export the file list in pa11y_targets so that we can make pa11y-ci scan only those changed files in the "Run pa11yci" step
+      - run:
+          name: Jekyll build
+          command: |
+            bundle exec jekyll build
+            echo "export PA11Y_TARGETS=$(cat pa11y_targets | base64 --wrap=0)" >> "$BASH_ENV"
+            source "$BASH_ENV"
       - run: npm run htmlproofer
       - run:
           name: Run rspec
           command: bundle exec rspec `pwd`/spec/
+      # Invoke pa11y-ci and pass in our list of changed files
       - run:
           name: Run pa11yci
-          command: ./serve-pa11yci && npm run pa11y-ci
+          command: npm run pa11y-ci -- $(echo $PA11Y_TARGETS | base64 --decode)
 workflows:
   version: 2
   commit:
diff --git a/_plugins/pa11y.rb b/_plugins/pa11y.rb
new file mode 100644
index 000000000..50022fcca
--- /dev/null
+++ b/_plugins/pa11y.rb
@@ -0,0 +1,191 @@
+=begin
+
+Determines which files to check with pa11y-ci per pull request.
+
+The current scope of pa11y testing is as follows:
+
+- Check any posts or pages whose content has changed.
+- Check any posts or pages using changed layouts.
+- If stylesheets have changed, check a sample of all page types:
+    - Three posts of each layout in _config.yml's `defaults`
+    - The first, last, and middle blog archive pages (/blog/page/:num)
+
+Previously, checking every page resulted in excessive CI runtimes.
+
+=end
+
+# Wraps Jekyll documents with convenience methods.
+#
+# @param path [String] The relative path of the source file, usually a
+#    Markdown file (.md)
+# @param layout [String] The name of the Jekyll document's layout
+# @param differ [Object, #changed_files] The collaborator which lists
+#   the files that were changed in the last commit.
+# @todo Are there cases when the document being checked has no layout?
+Document = Struct.new(:path, :layout, :destination, :differ) do
+  # @return [Boolean] Whether to scan this document in pa11y-ci
+  def to_scan?
+    source_changed? || layout_changed?
+  end
+
+  def source_changed?
+    changed_files.include?(path)
+  end
+
+  def layout_changed?
+    changed_files.include?("_layouts/#{layout}.html")
+  end
+
+  # Memoize these without having to memoize in the differ
+  def changed_files
+    @changed_files ||= differ.changed_files
+  end
+end
+
+class Differ
+  # @return [Array<String>] List of relative paths of files
+  def self.changed_files
+    @changed_files ||= %x[ #{list_files_command} ].to_s.split("\n").map(&:strip)
+  end
+
+  # @return [String] Text of a shell command to get listed files
+  def self.list_files_command
+    "git ls-files --modified"
+  end
+end
+
+# Gets the list of files changed in the last commit
+class CommitDiffer < Differ
+  def self.list_files_command
+    "git diff-tree --no-commit-id --name-only -r $(git rev-parse HEAD)"
+  end
+end
+
+
+class FileChecker
+
+  # Jekyll::Document#destination requires a root path parameter
+  # This constant replaces a mysterious empty string when
+  # calling #destination.
+  NO_ROOT_PATH = "".freeze
+
+  attr_reader :file, :pa11y_target_file, :differ
+
+  def initialize(file, pa11y_target_file, differ)
+    @file = file
+    @pa11y_target_file = pa11y_target_file
+    @differ = differ
+  end
+
+  def check_file_for_changes
+    document = Document.new(
+      file.relative_path,
+      file.data["layout"],
+      file.destination(NO_ROOT_PATH),
+      differ
+    )
+
+    if document.to_scan?
+      File.open(pa11y_target_file, 'a') { |f|
+        f.write(document.destination + "\n")
+      }
+    end
+  end
+end
+
+PA11Y_TARGET_FILE = ENV.fetch('PA11Y_TARGET_FILE') { 'pa11y_targets' }
+DIFFER = ENV.fetch("CI", false) ? CommitDiffer : Differ
+
+# Outputs posts and pages to scan to a file.
+Jekyll::Hooks.register :documents, :post_render do |doc|
+  FileChecker.new(doc, PA11Y_TARGET_FILE, DIFFER).check_file_for_changes
+end
+
+# The :documents hook above doesn't seem to touch the pages/ directory
+Jekyll::Hooks.register :pages, :post_render do |doc|
+  FileChecker.new(doc, PA11Y_TARGET_FILE, DIFFER).check_file_for_changes
+end
+
+# Produces a sample set of pages needed for a site-wide pa11y test.
+class SiteSampler
+
+  attr_reader :config, :site_files, :permalinks
+
+  def initialize(config, site_files=nil, permalinks=nil)
+    @config = config
+    @site_files = site_files || default_site_files
+    @permalinks = permalinks || default_permalinks
+  end
+
+  def pages
+    folders.flat_map { |folder| sample folder }
+  end
+
+  private
+
+  # Samples 3 files from a given folder, and its index
+  # @todo The matcher should change, not the list of files being returned
+  def sample(folder)
+    if ["_site/", "_site/blog/"].include?(folder)
+      [ File.join(folder, "index.html") ]
+    else
+      folder_regex = Regexp.new("^" + folder)
+      index_regex = Regexp.new("^" + File.join(folder, "index.html") + "$")
+      site_files.select do |file|
+        file.match?(folder_regex) || file.match?(index_regex)
+      end.reject {|filename| filename.match?(/.pdf$/i)}.sample(3)
+    end
+  end
+
+  def folders
+    permalinks.map do |permalink|
+      File.join("_site", parameterless_path(permalink), "/")
+    end.uniq.sort
+  end
+
+  # Default list of all the files in the site
+  def default_site_files
+    Dir.glob("_site/**/*").reject { |f| File.directory?(f) }
+  end
+
+  # Default list of all the permalink URL templates as given
+  # in the config document. These form the basis for the sampled
+  # pages later on.
+  def default_permalinks
+    [
+      config["jekyll-archives"]["permalinks"].values,
+      config["collections"].map { |_name, data| data["permalink"] },
+      config["paginate_path"],
+      "blog/"
+    ].flatten.uniq
+  end
+
+  # Follows the file paths until reaching a :param
+  def parameterless_path(path)
+    Pathname.new(path).each_filename.take_while { |x| !x.match? "^:" }.join("/")
+  end
+end
+
+
+# Changes to files in these directories trigger a global check
+# @todo Is there a way to get these more dynamically?
+SITEWIDE_FOLDERS = ["assets", "_includes", "_sass"]
+
+# If files have changed in any of the folders that trigger a site-wide scan,
+# sample folders across the site and add them to the list of files to check
+# with pa11y for a given Pull Request.
+# @todo Is there a way to unnest this code? Jekyll hooks don't allow
+#   early returns.
+Jekyll::Hooks.register :site, :post_render do |site|
+  # This rubular link shows an example of the generated regular expression
+  # and how it works against an imagined set of changed files
+  # https://rubular.com/r/nfFL9P69KHSBoY
+  global_files = Regexp.new SITEWIDE_FOLDERS.map { |x| "^#{x}" }.join("|")
+  if DIFFER.changed_files.grep(global_files).any?
+    File.open(PA11Y_TARGET_FILE, 'a') do |f|
+      SiteSampler.new(site.config).pages.each do |path|
+        f.write(path + "\n")
+      end
+    end
+  end
+end
diff --git a/package.json b/package.json
index 3f2fb9f1e..954602c39 100644
--- a/package.json
+++ b/package.json
@@ -13,7 +13,8 @@
     "federalist": "npm run uswds-build && mkdir ./.bundle && echo '---\nBUNDLE_GEMFILE: \"GemfileFederalist\"' > ./.bundle/config",
     "htmlproofer": "bundle exec htmlproofer _site --disable-external --allow-hash-href --ignore-empty-alt --ignore_urls \"18f@gsa.gov\" --no-enforce-https",
     "htmlproofer-local": "bundle exec htmlproofer _site --disable-external --allow-hash-href --ignore-empty-alt --ignore_urls \"localhost:4000\" --no-enforce-https --swap-urls \"^/site/:/\"",
-    "pa11y-ci": "pa11y-ci --sitemap http://localhost:4000/site/sitemap.xml --sitemap-exclude '.pdf|/google\\d'",
+    "pa11y-ci": "pa11y-ci",
+    "pa11y-sitemap": "pa11y-ci --sitemap http://localhost:4000/site/sitemap.xml --sitemap-exclude '.pdf|/google\\d'",
     "pa11y-limited": "pa11y-ci --sitemap http://localhost:4000/site/sitemap.xml --sitemap-exclude '.pdf|(/author/)|(/\\d+/)|/tags/|/google\\d'",
     "pa11y-local": "./killport 4000 && ./serve-pa11yci && npm run pa11y-ci",
     "restart": "rm -rf _site && ./serve",
diff --git a/spec/_plugins/pa11y_spec.rb b/spec/_plugins/pa11y_spec.rb
new file mode 100644
index 000000000..aa6b39efd
--- /dev/null
+++ b/spec/_plugins/pa11y_spec.rb
@@ -0,0 +1,101 @@
+require_relative '../../_plugins/pa11y'
+
+RSpec.describe Document do
+
+  let(:blog_path) { "_posts/2024-04-10-working-with-oracle-databases.md" }
+  let(:blog_layout) { "post" }
+  let(:blog_layout_path) { "_layouts/post.html" }
+  let(:destination_path) { "_site/2024/04/10/working-with-oracle-databases/index.html" }
+
+  # Stand-in for file diffing (GitDiffer), so tests don't rely on
+  # git history.
+  class TestDiffer
+    attr_reader :changed_files
+
+    def initialize(*paths)
+      @changed_files = *paths
+    end
+  end
+
+  # @example Use the convenience method
+  #   TestDiffer("_layouts/default.html", "_posts/2024-04-10-new-post.md")
+  def TestDiffer(*paths)
+    TestDiffer.new(*paths)
+  end
+
+  describe "#to_scan?" do
+    context "source files" do
+      let(:document) {
+        Document.new(blog_path, blog_layout, destination_path, test_differ)
+      }
+
+      context "with a matching changed source file" do
+        let(:test_differ) { TestDiffer(blog_path) }
+        it "returns true" do
+          expect(document.to_scan?).to be true
+        end
+      end
+
+      context "without a matching changed source file" do
+        let(:test_differ) { TestDiffer() }
+        it "returns false" do
+          expect(document.to_scan?).to be false
+        end
+      end
+
+      context "with a matching changed layout file" do
+        let(:test_differ) { TestDiffer(blog_layout_path) }
+        it "returns true" do
+          expect(document.to_scan?).to be true
+        end
+      end
+
+      context "without a matching changed layout file" do
+        let(:test_differ) { TestDiffer() }
+        it "returns false" do
+          expect(document.to_scan?).to be false
+        end
+      end
+
+      context "with a matching changed source file and changed layout file" do
+        let(:test_differ) { TestDiffer(blog_path, blog_layout_path) }
+        it "returns true" do
+          expect(document.to_scan?).to be true
+        end
+      end
+    end
+  end
+
+end
+
+
+RSpec.describe SiteSampler do
+
+  describe "#folders" do
+    context "with no site files or permalinks" do
+      let(:collector) { SiteSampler.new({}, [], []) }
+      it "lists no pages" do
+        expect(collector.pages.count).to eq(0)
+      end
+    end
+
+    context "with site files and permalinks" do
+      let(:site_files) { ["_site/blog/index.html", "_site/blog/page/2/index.html"] }
+      let(:permalinks) { ["blog/page/:num/", "blog/"] }
+      let(:collector) { SiteSampler.new({}, site_files, permalinks) }
+      it "lists pages" do
+        expect(collector.pages.count).to eq(2)
+      end
+    end
+    
+    context "with HTML and PDF files" do
+      let(:site_files) { ["_site/blog/index.html", "_site/blog/page/2/index.html", "_site/partnership-principles/18FPartnershipPrinciples.pdf"] }
+      let(:permalinks) { ["blog/page/:num/", "blog/"] }
+      let(:collector) { SiteSampler.new({}, site_files, permalinks) }
+      it "ignores PDF files" do
+        expect(collector.pages.count).to eq(2)
+      end
+    end
+
+  end
+end