diff --git a/lib/src/modules/default.nix b/lib/src/modules/default.nix
index 63105ef..a03a1f6 100644
--- a/lib/src/modules/default.nix
+++ b/lib/src/modules/default.nix
@@ -142,6 +142,18 @@ lib: {
           invalid = builtins.removeAttrs module lib.modules.VALID_KEYS;
         in
         invalid == { };
+
+      ## Check that an include object has the required attributes and otherwise
+      ## only specifies supported attributes.
+      ##
+      ## @type (Attrs | String | Path | Function) -> Bool
+      include =
+        include:
+        let
+          invalidKeys = builtins.removeAttrs include lib.modules.VALID_INCLUDE_KEYS;
+          namespaceIsString = include ? namespace -> builtins.isString include.namespace;
+        in
+        builtins.isAttrs include -> (include ? module && namespaceIsString && invalidKeys == { });
     };
 
     ## Modules only support certain keys at the root level. This list determines
@@ -159,6 +171,15 @@ lib: {
       "meta"
     ];
 
+    ## Include objects only support certain keys. This list determines the
+    ## valid attributes that users can supply.
+    ##
+    ## @type List String
+    VALID_INCLUDE_KEYS = [
+      "module"
+      "namespace"
+    ];
+
     ## Normalize a module to a standard structure. All other information will be
     ## lost in the conversion.
     ##
@@ -168,12 +189,62 @@ lib: {
       let
         invalid = builtins.removeAttrs module lib.modules.VALID_KEYS;
         invalidKeys = builtins.concatStringsSep ", " (builtins.attrNames invalid);
+        throwError = msg: builtins.throw ("Module ${key} (${file}) " + msg);
+
+        __key__ = builtins.toString module.__key__ or key;
+
+        normalizeIncludes =
+          includes:
+          let
+            normalized = lib.lists.mapWithIndex1 (
+              n: include:
+              let
+                invalid = builtins.removeAttrs module lib.modules.VALID_INCLUDE_KEYS;
+                invalidKeys = builtins.concatStringsSep ", " (builtins.attrNames invalid);
+                hasInvalidKeys = invalid != { };
+                throwError' = msg: throwError ("has invalid include at position ${builtins.toString n}" + msg);
+              in
+              if lib.modules.validate.include include then
+                if builtins.isAttrs include then
+                  {
+                    inherit (include) module;
+                    namespace = include.namespace or null;
+                  }
+                else
+                  {
+                    module = include;
+                    namespace = null;
+                  }
+              else if !include ? module then
+                {
+                  module = include;
+                  namespace = null;
+                }
+              else if hasInvalidKeys then
+                throwError' " with unsupported attribute(s): ${invalidKeys}"
+              else
+                throwError' ": namespace is not a string"
+            ) includes;
+
+            throwOnConflict =
+              let
+                filter =
+                  namespaces:
+                  { namespace, ... }:
+                  if namespace != null && builtins.elem namespace namespaces then
+                    throwError "declares several includes under the same namespace '${namespace}'"
+                  else
+                    namespaces ++ [ namespace ];
+              in
+              builtins.foldl' filter [ ] normalized;
+          in
+          builtins.seq throwOnConflict normalized;
       in
       if lib.modules.validate.keys module then
         {
-          __file__ = builtins.toString module.__file__ or file;
-          __key__ = builtins.toString module.__key__ or key;
-          includes = module.includes or [ ];
+          inherit __key__;
+          __file__ = builtins.toString (module.__file__ or file);
+          includes = normalizeIncludes (module.includes or [ ]);
           excludes = module.excludes or [ ];
           options = module.options or { };
           config =
@@ -201,7 +272,7 @@ lib: {
             withFreeform (withMeta base);
         }
       else
-        builtins.throw "Module `${key}` (${file}) has unsupported attribute(s): ${invalidKeys}";
+        throwError "has unsupported attribute(s): ${invalidKeys}";
 
     ## Convert a module that is either a function or an attribute set into
     ## a resolved attribute set. If the module was a function then it will
@@ -497,19 +568,36 @@ lib: {
         collect =
           let
             load =
-              args: file: key: module:
+              args: file: key: module':
               let
+                namespace = module'.namespace or null;
+                module = module'.module or module';
+
                 moduleFromValue = lib.modules.normalize file key (lib.modules.resolve key module args);
                 moduleFromPath = lib.modules.normalize (builtins.toString module) (builtins.toString module) (
                   lib.modules.resolve (builtins.toString module) (import module) args
                 );
               in
-              if builtins.isAttrs module || builtins.isFunction module then
-                moduleFromValue
-              else if builtins.isString module || builtins.isPath module then
-                moduleFromPath
+              if namespace == null then
+                if builtins.isAttrs module || builtins.isFunction module then
+                  moduleFromValue
+                else if builtins.isString module || builtins.isPath module then
+                  moduleFromPath
+                else
+                  builtins.throw "The provided module must be either an attribute set, function, or path but got ${builtins.typeOf module}"
+              # We don't resolve the module yet if we're under a namespace
               else
-                builtins.throw "The provided module must be either an attribute set, function, or path but got ${builtins.typeOf module}";
+                lib.modules.normalize file key {
+                  __key__ = "${key}:include-${namespace}";
+                  options.${namespace} = lib.options.create {
+                    description = "options and configuration included from ${namespace}";
+                    default.value = { };
+                    type = lib.types.submodules.of {
+                      modules = [ module ];
+                      description = "include ${namespace}";
+                    };
+                  };
+                };
 
             normalize =
               parentFile: parentKey: modules: args:
diff --git a/lib/src/modules/default.test.nix b/lib/src/modules/default.test.nix
index ff98eb5..cf37ea4 100644
--- a/lib/src/modules/default.test.nix
+++ b/lib/src/modules/default.test.nix
@@ -200,6 +200,62 @@ in
         in
         !value;
     };
+
+    "cannot mix include and module keys" =
+      let
+        # Simple backtracking algorithm to generate all combinations of a list's
+        # elements without ordering, in .
+        combinations =
+          set:
+          let
+            n = builtins.length set;
+            genIndices =
+              combinations':
+              let
+                prev = builtins.head combinations';
+                k = builtins.length prev;
+                first = builtins.head prev;
+                next' =
+                  # Generate the next combination of length k
+                  if k > 0 && first + 1 < n then
+                    [ (first + 1) ] ++ (builtins.tail prev)
+                  # We have generated all combinations of length k, generate the first
+                  # (trivial) combination of length k+1
+                  else
+                    builtins.genList (i: i) (k + 1);
+                next = builtins.deepSeq next' next';
+              in
+              # Generate the very first (trivial) combination
+              if combinations' == [ ] then
+                genIndices [ [ ] ]
+              # We have generated all combinations of length 0 to n, return
+              else if k >= n then
+                combinations'
+              # Generate the next combination and continue
+              else
+                genIndices ([ next ] ++ combinations');
+            select = indices: builtins.map (builtins.elemAt set) indices;
+          in
+          if set == [ ] then [ ] else builtins.map select (genIndices [ ]);
+
+        keys = combinations (lib.modules.VALID_KEYS ++ lib.modules.VALID_INCLUDE_KEYS);
+        sets = builtins.map (
+          names:
+          builtins.listToAttrs (
+            builtins.map (name: {
+              value = null;
+              inherit name;
+            }) names
+          )
+        ) keys;
+        validated = builtins.map (set: {
+          inherit set;
+          include = lib.modules.validate.include set;
+          module = lib.modules.validate.keys set;
+        }) sets;
+        bothValid = { include, module, ... }: include && module;
+      in
+      !builtins.any bothValid validated;
   };
 
   "normalize" = {
@@ -236,6 +292,91 @@ in
         };
       in
       actual == expected;
+
+    "includes" = {
+
+      "handles an empty list" =
+        let
+          expected = [ ];
+          actual = (lib.modules.normalize "/aux/example.nix" "example" { includes = [ ]; }).includes;
+        in
+        expected == actual;
+
+      "handles a mixed list" =
+        let
+          expected = [
+            {
+              module = { };
+              namespace = null;
+            }
+            {
+              module = { };
+              namespace = "a";
+            }
+          ];
+          actual =
+            (lib.modules.normalize "/aux/example.nix" "example" {
+              includes = [
+                { }
+                {
+                  module = { };
+                  namespace = "a";
+                }
+              ];
+            }).includes;
+        in
+        expected == actual;
+
+      "rejects conflicting namespaces" =
+        let
+          normalized = lib.modules.normalize "/aux/example.nix" "example" {
+            includes = [
+              {
+                module = { };
+                namespace = "a";
+              }
+              {
+                module = { };
+                namespace = "a";
+              }
+            ];
+          };
+        in
+        !(builtins.tryEval normalized.includes).success;
+
+      "allows multiple without namespace" =
+        let
+          normalized = lib.modules.normalize "/aux/example.nix" "example" {
+            includes = [
+              { }
+              { }
+            ];
+          };
+        in
+        (builtins.tryEval normalized.includes).success;
+
+      "handles multiple without namespace" =
+        let
+          expected = [
+            {
+              namespace = null;
+              module = { };
+            }
+            {
+              namespace = null;
+              module = { };
+            }
+          ];
+          actual =
+            (lib.modules.normalize "/aux/example.nix" "example" {
+              includes = [
+                { }
+                { }
+              ];
+            }).includes;
+        in
+        expected == actual;
+    };
   };
 
   "resolve" = {
@@ -699,5 +840,28 @@ in
         };
       in
       (evaluated.config.aux.message == evaluated.config.aux.message2) && evaluated.config.aux.exists;
+
+    "namespaced includes" =
+      let
+        expected = {
+          msg = "hello";
+          myinclude.msg = "hello";
+        };
+        evaluated = lib.modules.run {
+          modules = [
+            {
+              includes = [
+                {
+                  namespace = "myinclude";
+                  module.options.msg = lib.options.create { default.value = "hello"; };
+                }
+                { options.msg = lib.options.create { default.value = "hello"; }; }
+              ];
+            }
+          ];
+        };
+        actual = evaluated.config;
+      in
+      expected == actual;
   };
 }