diff --git a/spec/Section 3 -- Type System.md b/spec/Section 3 -- Type System.md
index b539b936e..8b9296ac0 100644
--- a/spec/Section 3 -- Type System.md	
+++ b/spec/Section 3 -- Type System.md	
@@ -148,12 +148,14 @@ type system where those operations begin.
 The {`query`} root operation type must be provided and must be an Object type.
 
 The {`mutation`} root operation type is optional; if it is not provided, the
-service does not support mutations. If it is provided, it must be an Object
-type.
+service does not support mutations (see
+[Operation Type Existence](#sec-Operation-Type-Existence)). If it is provided,
+it must be an Object type.
 
 Similarly, the {`subscription`} root operation type is also optional; if it is
-not provided, the service does not support subscriptions. If it is provided, it
-must be an Object type.
+not provided, the service does not support subscriptions (see
+[Operation Type Existence](#sec-Operation-Type-Existence))s. If it is provided,
+it must be an Object type.
 
 The {`query`}, {`mutation`}, and {`subscription`} root types must all be
 different types if provided.
diff --git a/spec/Section 5 -- Validation.md b/spec/Section 5 -- Validation.md
index 4eda8e7b4..3b1e0341d 100644
--- a/spec/Section 5 -- Validation.md	
+++ b/spec/Section 5 -- Validation.md	
@@ -129,6 +129,42 @@ extend type Dog {
 
 ## Operations
 
+### All Operation Definitions
+
+#### Operation Type Existence
+
+**Formal Specification**
+
+- For each operation definition {operation} in the document.
+- Let {operationRootType} be the root type in {schema} corresponding to the
+  type of {operation}.
+- {operationRootType} must exist.
+
+**Explanatory Text**
+
+Each operation must reference an operation type which has a valid root type
+in the schema.
+
+For example given the following schema:
+
+```graphql example
+type Query {
+  hello: String
+}
+```
+
+The following operation is valid:
+
+```graphql example
+query helloQuery { hello }
+```
+
+While the following operation is invalid:
+
+```graphql example
+mutation goodbyeMutation { goodbye }
+```
+
 ### Named Operation Definitions
 
 #### Operation Name Uniqueness