diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8acd80e6..68c9aa7c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,12 @@ The format is based on [Keep a Changelog], and this project adheres to
 [Keep a Changelog]: https://keepachangelog.com/en/1.0.0/
 [Semantic Versioning]: https://semver.org/spec/v2.0.0.html
 
+## [Unreleased]
+
+### Added
+
+- Add `Len()` to `NameCollection` and `TypeCollection` interfaces
+
 ## [0.7.1] - 2020-03-16
 
 ### Added
diff --git a/go.sum b/go.sum
index a286fd0a..c7b70d54 100644
--- a/go.sum
+++ b/go.sum
@@ -7,8 +7,6 @@ github.com/dogmatiq/dogma v0.6.3 h1:owkM3lOFmLmwaFpYogO46taVuPJU5sF5YApICZr4ljk=
 github.com/dogmatiq/dogma v0.6.3/go.mod h1:8TdjQ5jjV2DcCPb/jjHPgSrqU66H6092yMEIF5HGx0g=
 github.com/dogmatiq/iago v0.4.0 h1:57nZqVT34IZxtCZEW/RFif7DNUEjMXgevfr/Mmd0N8I=
 github.com/dogmatiq/iago v0.4.0/go.mod h1:fishMWBtzYcjgis6d873VTv9kFm/wHYLOzOyO9ECBDc=
-github.com/dogmatiq/linger v0.2.0 h1:WwpFy+0kwL0DNVFPx9tUuWxl2yZIALE5oFsjy8aWnKM=
-github.com/dogmatiq/linger v0.2.0/go.mod h1:vQPEiGNtb3Qy+1IdmdSMyLxr4d9vPChknnQkClQimGM=
 github.com/dogmatiq/linger v0.2.1 h1:ecVwiFlTcQuQ7ygQXH5bUPkWNve8n5BmVnCYMGMQ3zc=
 github.com/dogmatiq/linger v0.2.1/go.mod h1:vQPEiGNtb3Qy+1IdmdSMyLxr4d9vPChknnQkClQimGM=
 github.com/envoyproxy/go-control-plane v0.9.0/go.mod h1:YTl/9mNaCwkRvm6d1a2C3ymFceY/DCBVvsKhRF0iEA4=
diff --git a/message/name.go b/message/name.go
index 53aeabe2..6e63fbea 100644
--- a/message/name.go
+++ b/message/name.go
@@ -16,6 +16,9 @@ type NameCollection interface {
 	// HasM returns true if NameOf(m) is in the container.
 	HasM(m dogma.Message) bool
 
+	// Len returns the number of names in the collection.
+	Len() int
+
 	// Range invokes fn once for each name in the container.
 	//
 	// Iteration stops when fn returns false or once fn has been invoked for all
diff --git a/message/nameroles.go b/message/nameroles.go
index ea13ba26..308f0563 100644
--- a/message/nameroles.go
+++ b/message/nameroles.go
@@ -77,6 +77,11 @@ func (nr NameRoles) IsEqual(o NameRoles) bool {
 	return true
 }
 
+// Len returns the number of names in the collection.
+func (nr NameRoles) Len() int {
+	return len(nr)
+}
+
 // Range invokes fn once for each name in the container.
 //
 // Iteration stops when fn returns false or once fn has been invoked for all
diff --git a/message/nameroles_test.go b/message/nameroles_test.go
index cf8c917b..8fbf2c22 100644
--- a/message/nameroles_test.go
+++ b/message/nameroles_test.go
@@ -232,6 +232,17 @@ var _ = Describe("type NameRoles", func() {
 		)
 	})
 
+	Describe("func Len()", func() {
+		It("returns the number of names in the collection", func() {
+			nr := NameRoles{
+				MessageATypeName: CommandRole,
+				MessageBTypeName: EventRole,
+			}
+
+			Expect(nr.Len()).To(Equal(2))
+		})
+	})
+
 	Describe("func Range()", func() {
 		nr := NameRoles{
 			MessageATypeName: CommandRole,
diff --git a/message/nameset.go b/message/nameset.go
index f9ddac49..8a6364b1 100644
--- a/message/nameset.go
+++ b/message/nameset.go
@@ -164,6 +164,11 @@ func (s NameSet) IsEqual(o NameSet) bool {
 	return true
 }
 
+// Len returns the number of names in the collection.
+func (s NameSet) Len() int {
+	return len(s)
+}
+
 // Range invokes fn once for each name in the container.
 //
 // Iteration stops when fn returns false or once fn has been invoked for all
diff --git a/message/nameset_test.go b/message/nameset_test.go
index ed05a3fa..43cbb2a4 100644
--- a/message/nameset_test.go
+++ b/message/nameset_test.go
@@ -313,6 +313,17 @@ var _ = Describe("type NameSet", func() {
 		)
 	})
 
+	Describe("func Len()", func() {
+		It("returns the number of names in the collection", func() {
+			s := NewNameSet(
+				MessageATypeName,
+				MessageBTypeName,
+			)
+
+			Expect(s.Len()).To(Equal(2))
+		})
+	})
+
 	Describe("func Range()", func() {
 		s := NewNameSet(
 			MessageATypeName,
diff --git a/message/type.go b/message/type.go
index d67d2f6a..22048827 100644
--- a/message/type.go
+++ b/message/type.go
@@ -15,6 +15,9 @@ type TypeCollection interface {
 	// HasM returns true if TypeOf(m) is in the container.
 	HasM(m dogma.Message) bool
 
+	// Len returns the number of names in the collection.
+	Len() int
+
 	// Range invokes fn once for each type in the container.
 	//
 	// Iteration stops when fn returns false or once fn has been invoked for all
diff --git a/message/typeroles.go b/message/typeroles.go
index 46b691e7..7cd11b3d 100644
--- a/message/typeroles.go
+++ b/message/typeroles.go
@@ -77,6 +77,11 @@ func (tr TypeRoles) IsEqual(o TypeRoles) bool {
 	return true
 }
 
+// Len returns the number of types in the collection.
+func (tr TypeRoles) Len() int {
+	return len(tr)
+}
+
 // Range invokes fn once for each type in the container.
 //
 // Iteration stops when fn returns false or once fn has been invoked for all
diff --git a/message/typeroles_test.go b/message/typeroles_test.go
index 0deb3876..0f67e9a3 100644
--- a/message/typeroles_test.go
+++ b/message/typeroles_test.go
@@ -232,6 +232,17 @@ var _ = Describe("type TypeRoles", func() {
 		)
 	})
 
+	Describe("func Len()", func() {
+		It("returns the number of types in the collection", func() {
+			tr := TypeRoles{
+				MessageAType: CommandRole,
+				MessageBType: EventRole,
+			}
+
+			Expect(tr.Len()).To(Equal(2))
+		})
+	})
+
 	Describe("func Range()", func() {
 		tr := TypeRoles{
 			MessageAType: CommandRole,
diff --git a/message/typeset.go b/message/typeset.go
index c53fbc15..120c4895 100644
--- a/message/typeset.go
+++ b/message/typeset.go
@@ -164,6 +164,11 @@ func (s TypeSet) IsEqual(o TypeSet) bool {
 	return true
 }
 
+// Len returns the number of types in the collection.
+func (s TypeSet) Len() int {
+	return len(s)
+}
+
 // Range invokes fn once for each type in the container.
 //
 // Iteration stops when fn returns false or once fn has been invoked for all
diff --git a/message/typeset_test.go b/message/typeset_test.go
index 0e9021dd..c8a2ec27 100644
--- a/message/typeset_test.go
+++ b/message/typeset_test.go
@@ -313,6 +313,17 @@ var _ = Describe("type TypeSet", func() {
 		)
 	})
 
+	Describe("func Len()", func() {
+		It("returns the number of types in the collection", func() {
+			s := NewTypeSet(
+				MessageAType,
+				MessageBType,
+			)
+
+			Expect(s.Len()).To(Equal(2))
+		})
+	})
+
 	Describe("func Range()", func() {
 		s := NewTypeSet(
 			MessageAType,