diff --git a/src/numpydantic/interface/hdf5.py b/src/numpydantic/interface/hdf5.py
index ec32b0e..656273d 100644
--- a/src/numpydantic/interface/hdf5.py
+++ b/src/numpydantic/interface/hdf5.py
@@ -121,6 +121,10 @@ class H5Proxy:
                 else:
                     obj[key, self.field] = value
 
+    def __len__(self) -> int:
+        """self.shape[0]"""
+        return self.shape[0]
+
     def open(self, mode: str = "r") -> "h5py.Dataset":
         """
         Return the opened :class:`h5py.Dataset` object
diff --git a/src/numpydantic/interface/video.py b/src/numpydantic/interface/video.py
index 4f2048d..0ac7c66 100644
--- a/src/numpydantic/interface/video.py
+++ b/src/numpydantic/interface/video.py
@@ -180,6 +180,10 @@ class VideoProxy:
     def __getattr__(self, item: str):
         return getattr(self.video, item)
 
+    def __len__(self) -> int:
+        """Number of frames in the video"""
+        return self.shape[0]
+
 
 class VideoInterface(Interface):
     """
diff --git a/tests/fixtures.py b/tests/fixtures.py
index 347152c..e74c9a6 100644
--- a/tests/fixtures.py
+++ b/tests/fixtures.py
@@ -8,6 +8,7 @@ import numpy as np
 import pytest
 from pydantic import BaseModel, Field
 import zarr
+import cv2
 
 from numpydantic.interface.hdf5 import H5ArrayPath
 from numpydantic.interface.zarr import ZarrArrayPath
@@ -150,3 +151,34 @@ def zarr_array(tmp_output_dir_func) -> Path:
     array = zarr.open(str(file), mode="w", shape=(100, 100), chunks=(10, 10))
     array[:] = 0
     return file
+
+
+@pytest.fixture(scope="function")
+def avi_video(tmp_path) -> Callable[[Tuple[int, int], int, bool], Path]:
+    video_path = tmp_path / "test.avi"
+
+    def _make_video(shape=(100, 50), frames=10, is_color=True) -> Path:
+        writer = cv2.VideoWriter(
+            str(video_path),
+            cv2.VideoWriter_fourcc(*"RGBA"),  # raw video for testing purposes
+            30,
+            (shape[1], shape[0]),
+            is_color,
+        )
+        if is_color:
+            shape = (*shape, 3)
+
+        for i in range(frames):
+            # make fresh array every time bc opencv eats them
+            array = np.zeros(shape, dtype=np.uint8)
+            if not is_color:
+                array[i, i] = i
+            else:
+                array[i, i, :] = i
+            writer.write(array)
+        writer.release()
+        return video_path
+
+    yield _make_video
+
+    video_path.unlink(missing_ok=True)
diff --git a/tests/test_interface/conftest.py b/tests/test_interface/conftest.py
index e3bf0f7..63bdc4a 100644
--- a/tests/test_interface/conftest.py
+++ b/tests/test_interface/conftest.py
@@ -1,10 +1,12 @@
 import pytest
 
+from typing import Tuple, Callable
 import numpy as np
 import dask.array as da
 import zarr
+from pydantic import BaseModel
 
-from numpydantic import interface
+from numpydantic import interface, NDArray
 
 
 @pytest.fixture(
@@ -17,6 +19,7 @@ from numpydantic import interface
         (zarr.ones((10, 10)), interface.ZarrInterface),
         ("zarr_nested_array", interface.ZarrInterface),
         ("zarr_array", interface.ZarrInterface),
+        ("avi_video", interface.VideoInterface),
     ],
     ids=[
         "numpy_list",
@@ -26,9 +29,10 @@ from numpydantic import interface
         "zarr_memory",
         "zarr_nested",
         "zarr_array",
+        "video",
     ],
 )
-def interface_type(request):
+def interface_type(request) -> Tuple[NDArray, interface.Interface]:
     """
     Test cases for each interface's ``check`` method - each input should match the
     provided interface and that interface only
@@ -37,3 +41,20 @@ def interface_type(request):
         return (request.getfixturevalue(request.param[0]), request.param[1])
     else:
         return request.param
+
+
+@pytest.fixture()
+def all_interfaces(interface_type) -> BaseModel:
+    """
+    An instantiated version of each interface within a basemodel,
+    with the array in an `array` field
+    """
+    array, interface = interface_type
+    if isinstance(array, Callable):
+        array = array()
+
+    class MyModel(BaseModel):
+        array: NDArray
+
+    instance = MyModel(array=array)
+    return instance
diff --git a/tests/test_interface/test_dunder.py b/tests/test_interface/test_dunder.py
new file mode 100644
index 0000000..60f42d8
--- /dev/null
+++ b/tests/test_interface/test_dunder.py
@@ -0,0 +1,10 @@
+"""
+Tests for dunder methods on all interfaces
+"""
+
+
+def test_dunder_len(all_interfaces):
+    """
+    Each interface or proxy type should support __len__
+    """
+    assert len(all_interfaces.array) == all_interfaces.array.shape[0]
diff --git a/tests/test_interface/test_video.py b/tests/test_interface/test_video.py
index bfd7f7d..d5ef1b3 100644
--- a/tests/test_interface/test_video.py
+++ b/tests/test_interface/test_video.py
@@ -2,8 +2,6 @@
 Needs to be refactored to DRY, but works for now
 """
 
-import pdb
-
 import numpy as np
 import pytest
 
@@ -17,37 +15,6 @@ from numpydantic import dtype as dt
 from numpydantic.interface.video import VideoProxy
 
 
-@pytest.fixture(scope="function")
-def avi_video(tmp_path):
-    video_path = tmp_path / "test.avi"
-
-    def _make_video(shape=(100, 50), frames=10, is_color=True) -> Path:
-        writer = cv2.VideoWriter(
-            str(video_path),
-            cv2.VideoWriter_fourcc(*"RGBA"),  # raw video for testing purposes
-            30,
-            (shape[1], shape[0]),
-            is_color,
-        )
-        if is_color:
-            shape = (*shape, 3)
-
-        for i in range(frames):
-            # make fresh array every time bc opencv eats them
-            array = np.zeros(shape, dtype=np.uint8)
-            if not is_color:
-                array[i, i] = i
-            else:
-                array[i, i, :] = i
-            writer.write(array)
-        writer.release()
-        return video_path
-
-    yield _make_video
-
-    video_path.unlink(missing_ok=True)
-
-
 @pytest.mark.parametrize("input_type", [str, Path])
 def test_video_validation(avi_video, input_type):
     """Color videos should validate for normal uint8 shape specs"""