diff --git a/raillabel/format/_util.py b/raillabel/format/_util.py
index dfb567e..02f5e57 100644
--- a/raillabel/format/_util.py
+++ b/raillabel/format/_util.py
@@ -4,5 +4,13 @@
 from __future__ import annotations
 
 
+def _empty_list_to_none(collection: list | None) -> list | None:
+    if collection is None:
+        return None
+    if len(collection) == 0:
+        return None
+    return collection
+
+
 def _flatten_list(list_of_tuples: list[tuple]) -> list:
     return [item for tup in list_of_tuples for item in tup]
diff --git a/raillabel/format/frame.py b/raillabel/format/frame.py
index 686715e..91fb7c3 100644
--- a/raillabel/format/frame.py
+++ b/raillabel/format/frame.py
@@ -7,11 +7,19 @@
 from decimal import Decimal
 from uuid import UUID
 
-from raillabel.json_format import JSONFrame, JSONFrameProperties, JSONObjectData
-
+from raillabel.json_format import (
+    JSONAnnotations,
+    JSONFrame,
+    JSONFrameData,
+    JSONFrameProperties,
+    JSONObjectData,
+)
+
+from ._util import _empty_list_to_none
 from .bbox import Bbox
 from .cuboid import Cuboid
 from .num import Num
+from .object import Object
 from .poly2d import Poly2d
 from .poly3d import Poly3d
 from .seg3d import Seg3d
@@ -45,6 +53,19 @@ def from_json(cls, json: JSONFrame) -> Frame:
             annotations=_annotations_from_json(json.objects),
         )
 
+    def to_json(self, objects: dict[UUID, Object]) -> JSONFrame:
+        """Export this object into the RailLabel JSON format."""
+        return JSONFrame(
+            frame_properties=JSONFrameProperties(
+                timestamp=self.timestamp,
+                streams={
+                    sensor_id: sensor_ref.to_json() for sensor_id, sensor_ref in self.sensors.items()
+                },
+                frame_data=JSONFrameData(num=[num.to_json() for num in self.frame_data.values()]),
+            ),
+            objects=_objects_to_json(self.annotations, objects),
+        )
+
 
 def _timestamp_from_dict(frame_properties: JSONFrameProperties | None) -> Decimal | None:
     if frame_properties is None:
@@ -113,3 +134,65 @@ def _resolve_none_to_empty_list(optional_list: list | None) -> list:
     if optional_list is None:
         return []
     return optional_list
+
+
+def _objects_to_json(
+    annotations: dict[UUID, Bbox | Cuboid | Poly2d | Poly3d | Seg3d], objects: dict[UUID, Object]
+) -> dict[str, JSONObjectData] | None:
+    if len(annotations) == 0:
+        return None
+
+    object_data = {}
+
+    for ann_id, annotation in annotations.items():
+        object_id = str(annotation.object_id)
+
+        if object_id not in object_data:
+            object_data[object_id] = JSONObjectData(
+                object_data=JSONAnnotations(
+                    bbox=[],
+                    cuboid=[],
+                    poly2d=[],
+                    poly3d=[],
+                    vec=[],
+                )
+            )
+
+        json_annotation = annotation.to_json(ann_id, objects[UUID(object_id)].type)
+
+        if isinstance(annotation, Bbox):
+            object_data[object_id].object_data.bbox.append(json_annotation)  # type: ignore
+
+        elif isinstance(annotation, Cuboid):
+            object_data[object_id].object_data.cuboid.append(json_annotation)  # type: ignore
+
+        elif isinstance(annotation, Poly2d):
+            object_data[object_id].object_data.poly2d.append(json_annotation)  # type: ignore
+
+        elif isinstance(annotation, Poly3d):
+            object_data[object_id].object_data.poly3d.append(json_annotation)  # type: ignore
+
+        elif isinstance(annotation, Seg3d):
+            object_data[object_id].object_data.vec.append(json_annotation)  # type: ignore
+
+        else:
+            raise TypeError
+
+    for object_id in object_data:
+        object_data[object_id].object_data.bbox = _empty_list_to_none(
+            object_data[object_id].object_data.bbox
+        )
+        object_data[object_id].object_data.cuboid = _empty_list_to_none(
+            object_data[object_id].object_data.cuboid
+        )
+        object_data[object_id].object_data.poly2d = _empty_list_to_none(
+            object_data[object_id].object_data.poly2d
+        )
+        object_data[object_id].object_data.poly3d = _empty_list_to_none(
+            object_data[object_id].object_data.poly3d
+        )
+        object_data[object_id].object_data.vec = _empty_list_to_none(
+            object_data[object_id].object_data.vec
+        )
+
+    return object_data
diff --git a/raillabel/format/num.py b/raillabel/format/num.py
index acd2d62..e77d1c0 100644
--- a/raillabel/format/num.py
+++ b/raillabel/format/num.py
@@ -19,7 +19,10 @@ class Num:
     val: float
     "This is the value of the number object."
 
-    sensor_id: str | None
+    id: UUID | None = None
+    "The unique identifyer of the Num."
+
+    sensor_id: str | None = None
     "A reference to the sensor, this value is represented in."
 
     @classmethod
@@ -28,14 +31,15 @@ def from_json(cls, json: JSONNum) -> Num:
         return Num(
             name=json.name,
             val=json.val,
+            id=json.uid,
             sensor_id=json.coordinate_system,
         )
 
-    def to_json(self, uid: UUID) -> JSONNum:
+    def to_json(self) -> JSONNum:
         """Export this object into the RailLabel JSON format."""
         return JSONNum(
             name=self.name,
             val=self.val,
             coordinate_system=self.sensor_id,
-            uid=uid,
+            uid=self.id,
         )
diff --git a/tests/test_raillabel/format/conftest.py b/tests/test_raillabel/format/conftest.py
index 8ba0856..4f75bde 100644
--- a/tests/test_raillabel/format/conftest.py
+++ b/tests/test_raillabel/format/conftest.py
@@ -12,6 +12,7 @@
 from .test_metadata import metadata, metadata_json
 from .test_num import num, num_json, num_id
 from .test_object import (
+    objects,
     object_person,
     object_person_json,
     object_person_id,
diff --git a/tests/test_raillabel/format/test_frame.py b/tests/test_raillabel/format/test_frame.py
index 07cd720..1b85ba6 100644
--- a/tests/test_raillabel/format/test_frame.py
+++ b/tests/test_raillabel/format/test_frame.py
@@ -98,5 +98,10 @@ def test_from_json(frame, frame_json):
     assert actual == frame
 
 
+def test_to_json(frame, frame_json, objects):
+    actual = frame.to_json(objects)
+    assert actual == frame_json
+
+
 if __name__ == "__main__":
-    pytest.main([__file__, "-v"])
+    pytest.main([__file__, "-vv"])
diff --git a/tests/test_raillabel/format/test_num.py b/tests/test_raillabel/format/test_num.py
index 092c700..2779c86 100644
--- a/tests/test_raillabel/format/test_num.py
+++ b/tests/test_raillabel/format/test_num.py
@@ -29,11 +29,12 @@ def num_id() -> UUID:
 
 
 @pytest.fixture
-def num() -> Num:
+def num(num_id) -> Num:
     return Num(
         sensor_id="gps_imu",
         name="velocity",
         val=49.21321,
+        id=num_id,
     )
 
 
@@ -45,8 +46,8 @@ def test_from_json(num, num_json):
     assert actual == num
 
 
-def test_to_json(num, num_json, num_id):
-    actual = num.to_json(num_id)
+def test_to_json(num, num_json):
+    actual = num.to_json()
     assert actual == num_json
 
 
diff --git a/tests/test_raillabel/format/test_object.py b/tests/test_raillabel/format/test_object.py
index ae6b0da..9931826 100644
--- a/tests/test_raillabel/format/test_object.py
+++ b/tests/test_raillabel/format/test_object.py
@@ -13,6 +13,14 @@
 # == Fixtures =========================
 
 
+@pytest.fixture
+def objects(object_person, object_person_id, object_track, object_track_id) -> dict[UUID, Object]:
+    return {
+        object_person_id: object_person,
+        object_track_id: object_track,
+    }
+
+
 @pytest.fixture
 def object_person_json() -> JSONObject:
     return JSONObject(
diff --git a/tests/test_raillabel/format/test_poly2d.py b/tests/test_raillabel/format/test_poly2d.py
index 11da96a..c193197 100644
--- a/tests/test_raillabel/format/test_poly2d.py
+++ b/tests/test_raillabel/format/test_poly2d.py
@@ -21,7 +21,7 @@ def poly2d_json(
 ) -> JSONPoly2d:
     return JSONPoly2d(
         uid="013e7b34-62E5-435c-9412-87318c50f6d8",
-        name="rgb_middle__poly2d__person",
+        name="rgb_middle__poly2d__track",
         closed=True,
         mode="MODE_POLY2D_ABSOLUTE",
         val=point2d_json + another_point2d_json,
@@ -60,12 +60,12 @@ def test_from_json(poly2d, poly2d_json, object_track_id):
 
 
 def test_name(poly2d):
-    actual = poly2d.name("person")
-    assert actual == "rgb_middle__poly2d__person"
+    actual = poly2d.name("track")
+    assert actual == "rgb_middle__poly2d__track"
 
 
 def test_to_json(poly2d, poly2d_json, poly2d_id):
-    actual = poly2d.to_json(poly2d_id, object_type="person")
+    actual = poly2d.to_json(poly2d_id, object_type="track")
     assert actual == poly2d_json
 
 
diff --git a/tests/test_raillabel/format/test_poly3d.py b/tests/test_raillabel/format/test_poly3d.py
index 66288f5..e62d77e 100644
--- a/tests/test_raillabel/format/test_poly3d.py
+++ b/tests/test_raillabel/format/test_poly3d.py
@@ -21,7 +21,7 @@ def poly3d_json(
 ) -> JSONPoly3d:
     return JSONPoly3d(
         uid="0da87210-46F1-40e5-b661-20ea1c392f50",
-        name="lidar__poly3d__person",
+        name="lidar__poly3d__track",
         closed=True,
         val=point3d_json + another_point3d_json,
         coordinate_system="lidar",
@@ -59,12 +59,12 @@ def test_from_json(poly3d, poly3d_json, object_track_id):
 
 
 def test_name(poly3d):
-    actual = poly3d.name("person")
-    assert actual == "lidar__poly3d__person"
+    actual = poly3d.name("track")
+    assert actual == "lidar__poly3d__track"
 
 
 def test_to_json(poly3d, poly3d_json, poly3d_id):
-    actual = poly3d.to_json(poly3d_id, object_type="person")
+    actual = poly3d.to_json(poly3d_id, object_type="track")
     assert actual == poly3d_json