diff --git a/opentracing-api/src/field.rs b/opentracing-api/src/field.rs
new file mode 100644
index 0000000..eb45939
--- /dev/null
+++ b/opentracing-api/src/field.rs
@@ -0,0 +1,120 @@
+use std::fmt;
+use std::error::Error;
+use std::str::FromStr;
+
+const FIELD_ERROR_KIND: &str = "error.kind";
+const FIELD_ERROR_OBJECT: &str = "error.object";
+const FIELD_EVENT: &str = "event";
+const FIELD_MESSAGE: &str = "message";
+const FIELD_STACK: &str = "stack";
+
+/// The following log fields are recommended for instrumentors who are trying to capture more
+/// information about a logged event. Tracers may expose additional features based on these
+/// standardized data points.
+#[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
+pub enum Fields {
+    /// The type or "kind" of an error (only for event="error" logs). E.g., "Exception", "OSError".
+    ErrorKind,
+    /// The actual Throwable/Exception/Error object instance itself.
+    ErrorObject,
+    /// A stable identifier for some notable moment in the lifetime of a Span. For instance, a mutex
+    /// lock acquisition or release or the sorts of lifetime events in a browser page load described
+    /// in the Performance.timing specification. E.g., from Zipkin, "cs", "sr", "ss", or "cr". Or,
+    /// more generally, "initialized" or "timed out". For errors, "error".
+    Event,
+    /// A concise, human-readable, one-line message explaining the event. E.g., "Could not connect
+    /// to backend", "Cache invalidation succeeded".
+    Message,
+    /// A stack trace in platform-conventional format; may or may not pertain to an error.
+    Stack,
+}
+
+impl Fields {
+    /// Returns the string representation for the enum reference variant.
+    pub fn as_str(&self) -> &'static str {
+        match *self {
+            Fields::ErrorKind => FIELD_ERROR_KIND,
+            Fields::ErrorObject => FIELD_ERROR_OBJECT,
+            Fields::Event => FIELD_EVENT,
+            Fields::Message => FIELD_MESSAGE,
+            Fields::Stack => FIELD_STACK,
+        }
+    }
+}
+
+impl fmt::Display for Fields {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.as_str())
+    }
+}
+
+impl FromStr for Fields {
+    type Err = ParseFieldsError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s {
+            FIELD_ERROR_KIND => Ok(Fields::ErrorKind),
+            FIELD_ERROR_OBJECT => Ok(Fields::ErrorObject),
+            FIELD_EVENT => Ok(Fields::Event),
+            FIELD_MESSAGE => Ok(Fields::Message),
+            FIELD_STACK => Ok(Fields::Stack),
+            _ => Err(ParseFieldsError::UnknownField),
+        }
+    }
+}
+
+/// Describes errors which can happen while parsing into the `Fields` enum.
+#[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
+pub enum ParseFieldsError {
+    /// The provided field is not known.
+    UnknownField,
+}
+
+impl Error for ParseFieldsError {
+    fn description(&self) -> &str {
+        match *self {
+            ParseFieldsError::UnknownField => "Unknown Field",
+        }
+    }
+
+    fn cause(&self) -> Option<&Error> {
+        None
+    }
+}
+
+impl fmt::Display for ParseFieldsError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.description())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+
+    use super::*;
+
+    #[test]
+    fn test_field_as_str() {
+        assert_eq!("error.kind", Fields::ErrorKind.as_str());
+        assert_eq!("error.object", Fields::ErrorObject.as_str());
+        assert_eq!("event", Fields::Event.as_str());
+        assert_eq!("message", Fields::Message.as_str());
+        assert_eq!("stack", Fields::Stack.as_str());
+    }
+
+    #[test]
+    fn test_field_as_string() {
+        assert_eq!(String::from("error.kind"), Fields::ErrorKind.to_string());
+    }
+
+    #[test]
+    fn test_field_from_string() {
+        assert_eq!(Ok(Fields::Event), Fields::from_str("event"));
+        assert_eq!(Ok(Fields::Message), "message".parse());
+        assert_eq!(
+            Err(ParseFieldsError::UnknownField),
+            Fields::from_str("some_other_field")
+        );
+    }
+
+}
diff --git a/opentracing-api/src/lib.rs b/opentracing-api/src/lib.rs
index fb44d8d..60f0898 100644
--- a/opentracing-api/src/lib.rs
+++ b/opentracing-api/src/lib.rs
@@ -1 +1,7 @@
-#![doc(html_root_url = "https://docs.rs/opentracing-api/0.1.0")]
\ No newline at end of file
+#![doc(html_root_url = "https://docs.rs/opentracing-api/0.1.0")]
+
+mod tag;
+mod field;
+
+pub use tag::{Tags, ParseTagsError};
+pub use field::{Fields, ParseFieldsError};
diff --git a/opentracing-api/src/tag.rs b/opentracing-api/src/tag.rs
new file mode 100644
index 0000000..9f3123d
--- /dev/null
+++ b/opentracing-api/src/tag.rs
@@ -0,0 +1,199 @@
+use std::fmt;
+use std::error::Error;
+use std::str::FromStr;
+
+const TAG_SPAN_KIND_CLIENT: &str = "client";
+const TAG_SPAN_KIND_SERVER: &str = "server";
+const TAG_SPAN_KIND_PRODUCER: &str = "producer";
+const TAG_SPAN_KIND_CONSUMER: &str = "consumer";
+const TAG_HTTP_URL: &str = "http.url";
+const TAG_HTTP_STATUS_CODE: &str = "http.status_code";
+const TAG_HTTP_METHOD: &str = "http.method";
+const TAG_PEER_IPV4: &str = "peer.ipv4";
+const TAG_PEER_IPV6: &str = "peer.ipv6";
+const TAG_PEER_SERVICE: &str = "peer.service";
+const TAG_PEER_HOSTNAME: &str = "peer.hostname";
+const TAG_PEER_PORT: &str = "peer.port";
+const TAG_SAMPLING_PRIORITY: &str = "sampling.priority";
+const TAG_SPAN_KIND: &str = "span.kind";
+const TAG_COMPONENT: &str = "component";
+const TAG_ERROR: &str = "error";
+const TAG_DB_TYPE: &str = "db.type";
+const TAG_DB_INSTANCE: &str = "db.instance";
+const TAG_DB_USER: &str = "db.user";
+const TAG_DB_STATEMENT: &str = "db.statement";
+const TAG_MSGBUS_DEST: &str = "message_bus.destination";
+
+/// The following span tags are recommended for instrumentors who are trying to capture more
+/// semantic information about the spans. Tracers may expose additional features based on these
+/// standardized data points. Tag names follow a general structure of namespacing.
+///
+/// See also [Semantic Conventions](https://github.com/opentracing/specification/blob/master/semantic_conventions.md)
+#[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
+pub enum Tags {
+    /// A constant for setting the span kind to indicate that it represents a client span.
+    SpanKindClient,
+    /// A constant for setting the span kind to indicate that it represents a server span.
+    SpanKindServer,
+    /// A constant for setting the span kind to indicate that it represents a producer span,
+    /// in a messaging scenario.
+    SpanKindProducer,
+    /// A constant for setting the span kind to indicate that it represents a consumer span,
+    /// in a messaging scenario.
+    SpanKindConsumer,
+    /// HTTP_URL records the url of the incoming request.
+    HttpUrl,
+    /// HTTP_STATUS records the http status code of the response.
+    HttpStatus,
+    /// HTTP_METHOD records the http method. Case-insensitive.
+    HttpMethod,
+    /// PEER_HOST_IPV4 records IPv4 host address of the peer.
+    PeerHostIpv4,
+    /// PEER_HOST_IPV6 records the IPv6 host address of the peer.
+    PeerHostIpv6,
+    /// PEER_SERVICE records the service name of the peer.
+    PeerService,
+    /// PEER_HOSTNAME records the host name of the peer.
+    PeerHostname,
+    /// PEER_PORT records the port number of the peer.
+    PeerPort,
+    /// SAMPLING_PRIORITY determines the priority of sampling this Span.
+    SamplingPriority,
+    /// SPAN_KIND hints at the relationship between spans, e.g. client/server.
+    SpanKind,
+    /// COMPONENT is a low-cardinality identifier of the module, library, or
+    /// package that is instrumented.
+    Component,
+    /// ERROR indicates whether a Span ended in an error state.
+    Error,
+    /// DB_TYPE indicates the type of Database.
+    DbType,
+    /// DB_INSTANCE indicates the instance name of Database.
+    DbInstance,
+    /// DB_USER indicates the user name of Database, e.g. "readonly_user" or
+    /// "reporting_user"
+    DbUser,
+    /// DB_STATEMENT records a database statement for the given database type.
+    DbStatement,
+    /// MESSAGE_BUS_DESTINATION records an address at which messages can be
+    /// exchanged.
+    MessageBusDestination,
+}
+
+impl Tags {
+    /// Returns the string representation for the enum reference variant.
+    pub fn as_str(&self) -> &'static str {
+        match *self {
+            Tags::SpanKindClient => TAG_SPAN_KIND_CLIENT,
+            Tags::SpanKindServer => TAG_SPAN_KIND_SERVER,
+            Tags::SpanKindProducer => TAG_SPAN_KIND_PRODUCER,
+            Tags::SpanKindConsumer => TAG_SPAN_KIND_CONSUMER,
+            Tags::HttpUrl => TAG_HTTP_URL,
+            Tags::HttpStatus => TAG_HTTP_STATUS_CODE,
+            Tags::HttpMethod => TAG_HTTP_METHOD,
+            Tags::PeerHostIpv4 => TAG_PEER_IPV4,
+            Tags::PeerHostIpv6 => TAG_PEER_IPV6,
+            Tags::PeerService => TAG_PEER_SERVICE,
+            Tags::PeerHostname => TAG_PEER_HOSTNAME,
+            Tags::PeerPort => TAG_PEER_PORT,
+            Tags::SamplingPriority => TAG_SAMPLING_PRIORITY,
+            Tags::SpanKind => TAG_SPAN_KIND,
+            Tags::Component => TAG_COMPONENT,
+            Tags::Error => TAG_ERROR,
+            Tags::DbType => TAG_DB_TYPE,
+            Tags::DbInstance => TAG_DB_INSTANCE,
+            Tags::DbUser => TAG_DB_USER,
+            Tags::DbStatement => TAG_DB_STATEMENT,
+            Tags::MessageBusDestination => TAG_MSGBUS_DEST,
+        }
+    }
+}
+
+impl fmt::Display for Tags {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.as_str())
+    }
+}
+
+impl FromStr for Tags {
+    type Err = ParseTagsError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s {
+            TAG_SPAN_KIND_CLIENT => Ok(Tags::SpanKindClient),
+            TAG_SPAN_KIND_SERVER => Ok(Tags::SpanKindServer),
+            TAG_SPAN_KIND_PRODUCER => Ok(Tags::SpanKindProducer),
+            TAG_SPAN_KIND_CONSUMER => Ok(Tags::SpanKindConsumer),
+            TAG_HTTP_URL => Ok(Tags::HttpUrl),
+            TAG_HTTP_STATUS_CODE => Ok(Tags::HttpStatus),
+            TAG_HTTP_METHOD => Ok(Tags::HttpMethod),
+            TAG_PEER_IPV4 => Ok(Tags::PeerHostIpv4),
+            TAG_PEER_IPV6 => Ok(Tags::PeerHostIpv6),
+            TAG_PEER_SERVICE => Ok(Tags::PeerService),
+            TAG_PEER_HOSTNAME => Ok(Tags::PeerHostname),
+            TAG_PEER_PORT => Ok(Tags::PeerPort),
+            TAG_SAMPLING_PRIORITY => Ok(Tags::SamplingPriority),
+            TAG_SPAN_KIND => Ok(Tags::SpanKind),
+            TAG_COMPONENT => Ok(Tags::Component),
+            TAG_ERROR => Ok(Tags::Error),
+            TAG_DB_TYPE => Ok(Tags::DbType),
+            TAG_DB_INSTANCE => Ok(Tags::DbInstance),
+            TAG_DB_USER => Ok(Tags::DbUser),
+            TAG_DB_STATEMENT => Ok(Tags::DbStatement),
+            TAG_MSGBUS_DEST => Ok(Tags::MessageBusDestination),
+            _ => Err(ParseTagsError::UnknownTag),
+        }
+    }
+}
+
+/// Describes errors which can happen while parsing into the `Tags` enum.
+#[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
+pub enum ParseTagsError {
+    /// The provided tag is not known.
+    UnknownTag,
+}
+
+impl Error for ParseTagsError {
+    fn description(&self) -> &str {
+        match *self {
+            ParseTagsError::UnknownTag => "Unknown Tag",
+        }
+    }
+
+    fn cause(&self) -> Option<&Error> {
+        None
+    }
+}
+
+impl fmt::Display for ParseTagsError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.description())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+
+    use super::*;
+
+    #[test]
+    fn test_tag_as_str() {
+        assert_eq!("client", Tags::SpanKindClient.as_str());
+    }
+
+    #[test]
+    fn test_tag_as_string() {
+        assert_eq!(String::from("client"), Tags::SpanKindClient.to_string());
+    }
+
+    #[test]
+    fn test_tag_from_string() {
+        assert_eq!(Ok(Tags::Error), Tags::from_str("error"));
+        assert_eq!(Ok(Tags::PeerHostIpv4), "peer.ipv4".parse());
+        assert_eq!(
+            Err(ParseTagsError::UnknownTag),
+            Tags::from_str("some_other_field")
+        );
+    }
+
+}