diff --git a/extensions/3DTILES_metadata/README.md b/extensions/3DTILES_metadata/README.md
index 320a233a8..02d33746f 100644
--- a/extensions/3DTILES_metadata/README.md
+++ b/extensions/3DTILES_metadata/README.md
@@ -481,9 +481,9 @@ Metadata assigned to implicit tiles is stored in a more compact binary form. See
 
 Tiles may contain more than one content (see: [`3DTILES_multiple_contents`](../3DTILES_multiple_contents)), or multiple tiles may reference content sharing the same metadata. In these cases, metadata assigned to the tile would be inadequate or inefficient for describing tile contents. This extension allows content to be organized into collections, or "groups", and metadata may be associated with each group. Groups are useful for supporting metadata on only a subset of a tile's content, or for working with collections of contents as layers, e.g. to manage visibility or visual styling.
 
-Tile contents are assigned to groups, representing collections of content, by attaching a `3DTILES_metadata` extension to the content object and specifying its `group` property. Each content entity may be assigned only to a single group, but a single group may have any number of tile contents assigned to it.
+The tileset's root `3DTILES_metadata` extension must define a list of available groups, if any, under its `groups` property. Each group definition must specify its class (`class`) and an identifier (`id`) that uniquely identifies this group in the tileset. Within a `properties` dictionary, values for properties are given, encoded as JSON types according to the [JSON Format](../../specification/Metadata/README.md#json-format) specification.
 
-The tileset's root `3DTILES_metadata` extension must define a list of available groups, if any, under its `groups` property. Each group definition must specify its class (`class`). Within a `properties` dictionary, values for properties are given, encoded as JSON types according to the [JSON Format](../../specification/Metadata/README.md#json-format) specification.
+Tile contents are assigned to groups, representing collections of content, by attaching a `3DTILES_metadata` extension to the content object and specifying its `group` property to be the index of the group in the list of groups that was defined for the tileset. Each content entity may be assigned only to a single group, but a single group may have any number of tile contents assigned to it.
 
 > **Example:** The example below defines a custom "layer" class, where each of its two groups ("buildings" and "trees") are instances of the "layer" class associated with different "name", "color", and "priority" property values. The root tile defines two contents using `3DTILES_multiple_contents`, one content item belonging to each group.
 >
@@ -512,8 +512,9 @@ The tileset's root `3DTILES_metadata` extension must define a list of available
 >           }
 >         }
 >       },
->       "groups": {
->         "buildings": {
+>       "groups": [
+>         {
+>           "id": "buildings",
 >           "class": "layer",
 >           "properties": {
 >             "name": "Buildings Layer",
@@ -521,7 +522,8 @@ The tileset's root `3DTILES_metadata` extension must define a list of available
 >             "priority": 0
 >           }
 >         },
->         "trees": {
+>         {
+>           "id": "trees",
 >           "class": "layer",
 >           "properties": {
 >             "name": "Trees Layer",
@@ -529,7 +531,7 @@ The tileset's root `3DTILES_metadata` extension must define a list of available
 >             "priority": 1
 >           }
 >         }
->       }
+>       ]
 >     }
 >   },
 >   "root": {
@@ -538,11 +540,11 @@ The tileset's root `3DTILES_metadata` extension must define a list of available
 >         "content": [
 >           {
 >             "uri": "buildings.glb",
->             "extensions": {"3DTILES_metadata": {"group": "buildings"}}
+>             "extensions": {"3DTILES_metadata": {"group": 0 }}
 >           },
 >           {
 >             "uri": "trees.glb",
->             "extensions": {"3DTILES_metadata": {"group": "trees"}}
+>             "extensions": {"3DTILES_metadata": {"group": 1 }}
 >           }
 >         ]
 >       }
diff --git a/extensions/3DTILES_metadata/schema/content.3DTILES_metadata.schema.json b/extensions/3DTILES_metadata/schema/content.3DTILES_metadata.schema.json
index e2b9591bd..db28d81e2 100644
--- a/extensions/3DTILES_metadata/schema/content.3DTILES_metadata.schema.json
+++ b/extensions/3DTILES_metadata/schema/content.3DTILES_metadata.schema.json
@@ -11,8 +11,8 @@
     ],
     "properties": {
         "group": {
-            "type": "string",
-            "description": "The group this content belongs to. The value must correspond to a group ID declared in the `groups` dictionary."
+            "type": "integer",
+            "description": "The group this content belongs to. The value is an index into the array of `groups` that is defined for the containing tileset."
         },
         "class": {},
         "properties": {},
diff --git a/extensions/3DTILES_metadata/schema/group.schema.json b/extensions/3DTILES_metadata/schema/group.schema.json
index 17e06622d..fede9464b 100644
--- a/extensions/3DTILES_metadata/schema/group.schema.json
+++ b/extensions/3DTILES_metadata/schema/group.schema.json
@@ -10,9 +10,17 @@
         }
     ],
     "properties": {
+        "id": {
+            "type": "string",
+            "pattern": "^[a-zA-Z_][a-zA-Z0-9_]*$",
+            "description": "Unique identifier for the group within the tileset. This must be an alphanumeric identifier matching the regular expression `^[a-zA-Z_][a-zA-Z0-9_]*$`."
+        },
         "class": {},
         "properties": {},
         "extensions": {},
         "extras": {}
-    }
+    },
+    "required": [
+        "id"
+    ]
 }
\ No newline at end of file
diff --git a/extensions/3DTILES_metadata/schema/tileset.3DTILES_metadata.schema.json b/extensions/3DTILES_metadata/schema/tileset.3DTILES_metadata.schema.json
index 5d49b6794..16b2955b1 100644
--- a/extensions/3DTILES_metadata/schema/tileset.3DTILES_metadata.schema.json
+++ b/extensions/3DTILES_metadata/schema/tileset.3DTILES_metadata.schema.json
@@ -32,12 +32,12 @@
             "description": "An object containing statistics about entities."
         },
         "groups": {
-            "type": "object",
-            "description": "A dictionary, where each key is a group ID and each value is an object defining the group. Group IDs must be alphanumeric identifiers matching the regular expression `^[a-zA-Z_][a-zA-Z0-9_]*$`.",
-            "minProperties": 1,
-            "additionalProperties": {
+            "type": "array",
+            "description": "An array of groups.",
+            "items": {
                 "$ref": "group.schema.json"
-            }
+            },
+            "minItems": 1
         },
         "tileset": {
             "allOf": [
diff --git a/next/REVISION_HISTORY.md b/next/REVISION_HISTORY.md
index 29c857bed..f46a885f2 100644
--- a/next/REVISION_HISTORY.md
+++ b/next/REVISION_HISTORY.md
@@ -74,6 +74,7 @@
   * Removed incomplete styling section
   * Recommend `_*` prefix for application-specific summary statistics
   * Removed `name` and `description` from entity schemas. Entities should use properties with equivalent semantics instead.
+  * The `groups` that are defined in a tileset are now given as an array. Each group has a unique identifier.
 * Updated `3DTILES_implicit_tiling` extension
   * Updated to draft version `1.0.0`
   * Updated property table definition. See notes above.