diff --git a/smithy-model/src/main/java/software/amazon/smithy/model/loader/AstModelLoader.java b/smithy-model/src/main/java/software/amazon/smithy/model/loader/AstModelLoader.java
index 3f0f70c1168..98d4ce476b3 100644
--- a/smithy-model/src/main/java/software/amazon/smithy/model/loader/AstModelLoader.java
+++ b/smithy-model/src/main/java/software/amazon/smithy/model/loader/AstModelLoader.java
@@ -76,19 +76,19 @@ final class AstModelLoader {
 
     private static final List<String> TOP_LEVEL_PROPERTIES = ListUtils.of("smithy", SHAPES, METADATA);
     private static final List<String> APPLY_PROPERTIES = ListUtils.of(TYPE, TRAITS);
-    private static final List<String> SIMPLE_PROPERTY_NAMES = ListUtils.of(TYPE, TRAITS);
+    private static final List<String> SIMPLE_PROPERTY_NAMES = ListUtils.of(TYPE, TRAITS, MIXINS);
     private static final List<String> NAMED_MEMBER_SHAPE_PROPERTY_NAMES = ListUtils.of(TYPE, MEMBERS, TRAITS, MIXINS);
-    private static final List<String> COLLECTION_PROPERTY_NAMES = ListUtils.of(TYPE, "member", TRAITS);
-    private static final List<String> MAP_PROPERTY_NAMES = ListUtils.of(TYPE, "key", "value", TRAITS);
+    private static final List<String> COLLECTION_PROPERTY_NAMES = ListUtils.of(TYPE, "member", TRAITS, MIXINS);
+    private static final List<String> MAP_PROPERTY_NAMES = ListUtils.of(TYPE, "key", "value", TRAITS, MIXINS);
     private static final Set<String> MEMBER_PROPERTIES = SetUtils.of(TARGET, TRAITS);
     private static final Set<String> REFERENCE_PROPERTIES = SetUtils.of(TARGET);
     private static final Set<String> OPERATION_PROPERTY_NAMES = SetUtils.of(
-            TYPE, "input", "output", ERRORS, TRAITS);
+            TYPE, "input", "output", ERRORS, TRAITS, MIXINS);
     private static final Set<String> RESOURCE_PROPERTIES = SetUtils.of(
             TYPE, "create", "read", "update", "delete", "list", "put",
-            "identifiers", "resources", "operations", "collectionOperations", "properties", TRAITS);
+            "identifiers", "resources", "operations", "collectionOperations", "properties", TRAITS, MIXINS);
     private static final Set<String> SERVICE_PROPERTIES = SetUtils.of(
-            TYPE, "version", "operations", "resources", "rename", ERRORS, TRAITS);
+            TYPE, "version", "operations", "resources", "rename", ERRORS, TRAITS, MIXINS);
 
     private final Version modelVersion;
     private final ObjectNode model;
diff --git a/smithy-model/src/test/java/software/amazon/smithy/model/loader/AstModelLoaderTest.java b/smithy-model/src/test/java/software/amazon/smithy/model/loader/AstModelLoaderTest.java
index 561f0238072..51d1d29a792 100644
--- a/smithy-model/src/test/java/software/amazon/smithy/model/loader/AstModelLoaderTest.java
+++ b/smithy-model/src/test/java/software/amazon/smithy/model/loader/AstModelLoaderTest.java
@@ -27,20 +27,83 @@ public class AstModelLoaderTest {
     @Test
     public void failsToLoadPropertiesFromV1() {
         ValidatedResult<Model> model = Model.assembler()
-                .addImport(getClass().getResource("invalid/properties-v2-only.json"))
-                .assemble();
+            .addImport(getClass().getResource("invalid/properties-v2-only.json"))
+            .assemble();
         assertEquals(1, model.getValidationEvents(Severity.ERROR).size());
         assertTrue(model.getValidationEvents(Severity.ERROR).get(0).getMessage()
-                .contains("Resource properties can only be used with Smithy version 2 or later."));
+            .contains("Resource properties can only be used with Smithy version 2 or later."));
     }
 
     @Test
     public void doesNotFailOnEmptyApply() {
         // Empty apply statements are pointless but shouldn't break the loader.
         Model.assembler()
-                .addImport(getClass().getResource("ast-empty-apply-1.json"))
-                .addImport(getClass().getResource("ast-empty-apply-2.json"))
-                .assemble()
-                .unwrap();
+            .addImport(getClass().getResource("ast-empty-apply-1.json"))
+            .addImport(getClass().getResource("ast-empty-apply-2.json"))
+            .assemble()
+            .unwrap();
+    }
+
+    @Test
+    public void allowsMixinsOnOperationsWithoutWarningOrError() {
+        ValidatedResult<Model> model = Model.assembler()
+            .addImport(getClass().getResource("mixins/operation-mixins.json"))
+            .assemble();
+        assertEquals(0, model.getValidationEvents(Severity.WARNING).size());
+        assertEquals(0, model.getValidationEvents(Severity.ERROR).size());
+    }
+
+    @Test
+    public void allowsMixinsOnResourcesWithoutWarningOrError() {
+        ValidatedResult<Model> model = Model.assembler()
+            .addImport(getClass().getResource("mixins/resource-mixins.json"))
+            .assemble();
+        assertEquals(0, model.getValidationEvents(Severity.WARNING).size());
+        assertEquals(0, model.getValidationEvents(Severity.ERROR).size());
+    }
+
+    @Test
+    public void allowsMixinsOnServiceWithoutWarningOrError() {
+        ValidatedResult<Model> model = Model.assembler()
+            .addImport(getClass().getResource("mixins/service-mixins.json"))
+            .assemble();
+        assertEquals(0, model.getValidationEvents(Severity.WARNING).size());
+        assertEquals(0, model.getValidationEvents(Severity.ERROR).size());
+    }
+
+    @Test
+    public void allowsMixinsOnStructuresWithoutWarningOrError() {
+        ValidatedResult<Model> model = Model.assembler()
+            .addImport(getClass().getResource("mixins/structure-mixins.json"))
+            .assemble();
+        assertEquals(0, model.getValidationEvents(Severity.WARNING).size());
+        assertEquals(0, model.getValidationEvents(Severity.ERROR).size());
+    }
+
+    @Test
+    public void allowsMixinsOnUnionsWithoutWarningOrError() {
+        ValidatedResult<Model> model = Model.assembler()
+            .addImport(getClass().getResource("mixins/union-mixins.json"))
+            .assemble();
+        assertEquals(0, model.getValidationEvents(Severity.WARNING).size());
+        assertEquals(0, model.getValidationEvents(Severity.ERROR).size());
+    }
+
+    @Test
+    public void allowsMixinsOnSimpleShapesWithoutWarningOrError() {
+        ValidatedResult<Model> model = Model.assembler()
+            .addImport(getClass().getResource("mixins/simple-shape-mixins.json"))
+            .assemble();
+        assertEquals(0, model.getValidationEvents(Severity.WARNING).size());
+        assertEquals(0, model.getValidationEvents(Severity.ERROR).size());
+    }
+
+    @Test
+    public void allowsMixinsOnCollectionShapesWithoutWarningOrError() {
+        ValidatedResult<Model> model = Model.assembler()
+            .addImport(getClass().getResource("mixins/collection-mixins.json"))
+            .assemble();
+        assertEquals(0, model.getValidationEvents(Severity.WARNING).size());
+        assertEquals(0, model.getValidationEvents(Severity.ERROR).size());
     }
 }
diff --git a/smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/loader/map-warns-additional-properties.errors b/smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/loader/map-warns-additional-properties.errors
index b430ae3b444..9650851199a 100644
--- a/smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/loader/map-warns-additional-properties.errors
+++ b/smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/loader/map-warns-additional-properties.errors
@@ -1 +1 @@
-[WARNING] smithy.example#Hi: Expected an object with possible properties of `key`, `traits`, `type`, `value`, but found additional properties: `foo` | Model
+[WARNING] smithy.example#Hi: Expected an object with possible properties of `key`, `mixins`, `traits`, `type`, `value`, but found additional properties: `foo` | Model
diff --git a/smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/loader/shapes-warn-additional-properties.errors b/smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/loader/shapes-warn-additional-properties.errors
index f3626829572..8ada31b1540 100644
--- a/smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/loader/shapes-warn-additional-properties.errors
+++ b/smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/loader/shapes-warn-additional-properties.errors
@@ -1 +1 @@
-[WARNING] smithy.example#Hi: Expected an object with possible properties of `traits`, `type`, but found additional properties: `invalid` | Model
+[WARNING] smithy.example#Hi: Expected an object with possible properties of `mixins`, `traits`, `type`, but found additional properties: `invalid` | Model
diff --git a/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/collection-mixins.json b/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/collection-mixins.json
new file mode 100644
index 00000000000..faad2450d79
--- /dev/null
+++ b/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/collection-mixins.json
@@ -0,0 +1,50 @@
+{
+  "smithy": "2.0",
+  "shapes": {
+    "smithy.example#MyList": {
+      "type": "list",
+      "mixins": [
+        {
+          "target": "smithy.example#MyListMixin"
+        }
+      ]
+    },
+    "smithy.example#MyListMixin": {
+      "type": "list",
+      "member": {
+        "target": "smithy.api#String"
+      },
+      "traits": {
+        "smithy.api#length": {
+          "min": 2,
+          "max": 4
+        },
+        "smithy.api#mixin": {}
+      }
+    },
+    "smithy.example#MyMap": {
+      "type": "map",
+      "mixins": [
+        {
+          "target": "smithy.example#MyMapMixin"
+        }
+      ]
+    },
+    "smithy.example#MyMapMixin": {
+      "type": "map",
+      "key": {
+        "target": "smithy.api#String"
+      },
+      "value": {
+        "target": "smithy.api#String"
+      },
+      "traits": {
+        "smithy.api#length": {
+          "min": 1,
+          "max": 4
+        },
+        "smithy.api#mixin": {}
+      }
+    }
+  }
+}
\ No newline at end of file
diff --git a/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/operation-mixins.json b/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/operation-mixins.json
new file mode 100644
index 00000000000..cdb781b55cc
--- /dev/null
+++ b/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/operation-mixins.json
@@ -0,0 +1,77 @@
+{
+  "smithy": "2.0",
+  "shapes": {
+    "example.weather.errors#ThrottlingException": {
+      "type": "structure",
+      "members": {},
+      "traits": {
+        "smithy.api#error": "server"
+      }
+    },
+    "example.weather.errors#ValidationException": {
+      "type": "structure",
+      "members": {},
+      "traits": {
+        "smithy.api#error": "client"
+      }
+    },
+    "example.weather.mixins#OperationMixin": {
+      "type": "operation",
+      "input": {
+        "target": "smithy.api#Unit"
+      },
+      "output": {
+        "target": "smithy.api#Unit"
+      },
+      "errors": [
+        {
+          "target": "example.weather.errors#ThrottlingException"
+        },
+        {
+          "target": "example.weather.errors#ValidationException"
+        }
+      ],
+      "traits": {
+        "smithy.api#mixin": {}
+      }
+    },
+    "example.weather.operations#OperationWithMixin": {
+      "type": "operation",
+      "mixins": [
+        {
+          "target": "example.weather.mixins#OperationMixin"
+        }
+      ],
+      "input": {
+        "target": "example.weather.operations#OperationWithMixinInput"
+      },
+      "output": {
+        "target": "smithy.api#Unit"
+      },
+      "traits": {
+        "smithy.api#http": {
+          "method": "POST",
+          "uri": "/my/resource/uri/{myInputField}"
+        }
+      }
+    },
+    "example.weather.operations#OperationWithMixinInput": {
+      "type": "structure",
+      "members": {
+        "myInputField": {
+          "target": "smithy.api#String",
+          "traits": {
+            "smithy.api#httpLabel": {},
+            "smithy.api#required": {}
+          }
+        },
+        "other": {
+          "target": "smithy.api#String"
+        }
+      },
+      "traits": {
+        "smithy.api#input": {}
+      }
+    }
+  }
+}
\ No newline at end of file
diff --git a/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/resource-mixins.json b/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/resource-mixins.json
new file mode 100644
index 00000000000..c567ff296ff
--- /dev/null
+++ b/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/resource-mixins.json
@@ -0,0 +1,46 @@
+{
+  "smithy": "2.0",
+  "shapes": {
+    "example.weather.resources#DummyOperation": {
+      "type": "operation",
+      "input": {
+        "target": "example.weather.resources#DummyOperationInput"
+      },
+      "output": {
+        "target": "smithy.api#Unit"
+      },
+      "traits": {
+      }
+    },
+    "example.weather.resources#DummyOperationInput": {
+      "type": "structure",
+      "members": {
+        "dummyInput": {
+          "target": "smithy.api#String"
+        }
+      },
+      "traits": {
+        "smithy.api#input": {}
+      }
+    },
+    "example.weather.mixins#ResourceMixin": {
+      "type": "resource",
+      "traits": {
+        "smithy.api#mixin": {}
+      }
+    },
+    "example.weather.resources#ResourceWithMixin": {
+      "type": "resource",
+      "mixins": [
+        {
+          "target": "example.weather.mixins#ResourceMixin"
+        }
+      ],
+      "operations": [
+        {
+          "target": "example.weather.resources#DummyOperation"
+        }
+      ]
+    }
+  }
+}
\ No newline at end of file
diff --git a/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/service-mixins.json b/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/service-mixins.json
new file mode 100644
index 00000000000..fde05b02050
--- /dev/null
+++ b/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/service-mixins.json
@@ -0,0 +1,44 @@
+{
+  "smithy": "2.0",
+  "shapes": {
+    "smithy.example#MyDummyOperation": {
+      "type": "operation",
+      "input": {
+        "target": "smithy.example#MyDummyOperationInput"
+      },
+      "output": {
+        "target": "smithy.api#Unit"
+      }
+    },
+    "smithy.example#MyDummyOperationInput": {
+      "type": "structure",
+      "members": {
+        "foo": {
+          "target": "smithy.api#String"
+        }
+      },
+      "traits": {
+        "smithy.api#input": {}
+      }
+    },
+    "smithy.example#MyMixin": {
+      "type": "service",
+      "operations": [
+        {
+          "target": "smithy.example#MyDummyOperation"
+        }
+      ],
+      "traits": {
+        "smithy.api#mixin": {}
+      }
+    },
+    "smithy.example#MyService": {
+      "type": "service",
+      "mixins": [
+        {
+          "target": "smithy.example#MyMixin"
+        }
+      ]
+    }
+  }
+}
\ No newline at end of file
diff --git a/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/simple-shape-mixins.json b/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/simple-shape-mixins.json
new file mode 100644
index 00000000000..3043d5e93d4
--- /dev/null
+++ b/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/simple-shape-mixins.json
@@ -0,0 +1,40 @@
+{
+  "smithy": "2.0",
+  "shapes": {
+    "smithy.example#MyIntMixin": {
+      "type": "integer",
+      "traits": {
+        "smithy.api#mixin": {},
+        "smithy.api#range": {
+          "min": 1
+        }
+      }
+    },
+    "smithy.example#MyStringMixin": {
+      "type": "string",
+      "traits": {
+        "smithy.api#length": {
+          "min": 2,
+          "max": 4
+        },
+        "smithy.api#mixin": {}
+      }
+    },
+    "smithy.example#MyTestInt": {
+      "type": "integer",
+      "mixins": [
+        {
+          "target": "smithy.example#MyIntMixin"
+        }
+      ]
+    },
+    "smithy.example#MyTestString": {
+      "type": "string",
+      "mixins": [
+        {
+          "target": "smithy.example#MyStringMixin"
+        }
+      ]
+    }
+  }
+}
\ No newline at end of file
diff --git a/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/structure-mixins.json b/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/structure-mixins.json
new file mode 100644
index 00000000000..fc001548850
--- /dev/null
+++ b/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/structure-mixins.json
@@ -0,0 +1,29 @@
+{
+  "smithy": "2.0",
+  "shapes": {
+    "smithy.example#Baz": {
+      "type": "structure",
+      "mixins": [
+        {
+          "target": "smithy.example#MyMixin"
+        }
+      ],
+      "members": {
+        "foo": {
+          "target": "smithy.api#String"
+        }
+      }
+    },
+    "smithy.example#MyMixin": {
+      "type": "structure",
+      "members": {
+        "bar": {
+          "target": "smithy.api#String"
+        }
+      },
+      "traits": {
+        "smithy.api#mixin": {}
+      }
+    }
+  }
+}
\ No newline at end of file
diff --git a/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/union-mixins.json b/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/union-mixins.json
new file mode 100644
index 00000000000..c4c7e186025
--- /dev/null
+++ b/smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/union-mixins.json
@@ -0,0 +1,32 @@
+{
+  "smithy": "2.0",
+  "shapes": {
+    "smithy.example#MyMixin": {
+      "type": "union",
+      "members": {
+        "bar": {
+          "target": "smithy.api#String"
+        }
+      },
+      "traits": {
+        "smithy.api#mixin": {}
+      }
+    },
+    "smithy.example#MyUnion": {
+      "type": "union",
+      "mixins": [
+        {
+          "target": "smithy.example#MyMixin"
+        }
+      ],
+      "members": {
+        "baz": {
+          "target": "smithy.api#String"
+        }
+      },
+      "traits": {
+        "smithy.api#mixin": {}
+      }
+    }
+  }
+}
\ No newline at end of file