diff --git a/spec.md b/spec.md
index 7f5d5b465..3d250ee0e 100644
--- a/spec.md
+++ b/spec.md
@@ -190,18 +190,28 @@ within the same JSON object.
 ### source
 
 - Type: `URI-reference`
-- Description: This describes the event producer. Often this will include
-  information such as the type of the event source, the organization publishing
-  the event, the process that produced the event, and some unique identifiers.
-  The exact syntax and semantics behind the data encoded in the URI is event
-  producer defined.
+- Description: Identifies the event producer. Often this will include
+  information such as the type of the event source, the organization
+  publishing the event, the process that produced the event, and some unique
+  identifiers. The exact syntax and semantics behind the data encoded in the URI
+  is event producer defined.
+
+  An application MUST assign a distinct `source` to each distinct producer.
+  The application MAY use UUIDs, URNs, DNS authorities or an application-specific
+  scheme to create unique identifiers.
+
 - Constraints:
   - REQUIRED
 - Examples
-  - https://github.com/cloudevents/spec/pull
-  - /cloudevents/spec/pull
-  - urn:event:from:myapi/resource
-  - mailto:cncf-wg-serverless@lists.cncf.io
+  - Internet-wide unique URI with a DNS authority.
+    - https://github.com/cloudevents/spec/pull/123
+    - mailto:cncf-wg-serverless@lists.cncf.io
+  - Universally-unique URN with a UUID:
+    -  urn:uuid:6e8bc430-9c3a-11d9-9669-0800200c9a66
+  - Application-specific identifiers
+    - /cloudevents/spec/pull/123
+    - /sensors/tn-1234567/alerts
+    - 1-555-123-4567
 
 ### subject
 
@@ -236,14 +246,20 @@ within the same JSON object.
 ### id
 
 - Type: `String`
-- Description: ID of the event. The semantics of this string are explicitly
-  undefined to ease the implementation of producers. Enables deduplication.
+- Description: Optionally identifies an event for de-duplication.
+  Absent if de-duplication is not required.
+  If present, the `source` and `id` attributes together uniquely identify the event.
 - Examples:
+  - An event counter maintained by the producer
   - A database commit ID
+  - A UUID
 - Constraints:
-  - REQUIRED
-  - MUST be a non-empty string
-  - MUST be unique within the scope of the producer
+  - OPTIONAL
+    - Producers MAY omit `id` if de-duplication is not required.
+    - Consumers MUST process events lacking `id` as non-duplicate events.
+  - If present, MUST be a non-empty string.
+    - Producers MUST ensure each distinct event from the same `source` has a unique `id`  If a duplicate event is re-sent due to a network error, it MAY have the same `id`.
+    - Consumers MAY assume that events with matching `source` and `id` are duplicates.
 
 ### time