diff --git a/test/test_specs.py b/test/test_specs.py
index 7472a2b8e08..aaf30a0906e 100644
--- a/test/test_specs.py
+++ b/test/test_specs.py
@@ -2045,7 +2045,7 @@ def test_stack_unboundeddiscrete_rand(self, shape, stack_dim):
         shape = (*shape,)
         c1 = UnboundedDiscreteTensorSpec(shape=shape)
         c2 = c1.clone()
-        c = torch.stack([c1, c2], 0)
+        c = torch.stack([c1, c2], stack_dim)
         r = c.rand()
         assert r.shape == c.shape
 
@@ -2053,10 +2053,28 @@ def test_stack_unboundeddiscrete_zero(self, shape, stack_dim):
         shape = (*shape,)
         c1 = UnboundedDiscreteTensorSpec(shape=shape)
         c2 = c1.clone()
-        c = torch.stack([c1, c2], 0)
+        c = torch.stack([c1, c2], stack_dim)
         r = c.zero()
         assert r.shape == c.shape
 
+    def test_to_numpy(self, shape, stack_dim):
+        c1 = BoundedTensorSpec(-1, 1, shape=shape, dtype=torch.float64)
+        c2 = BoundedTensorSpec(-1, 1, shape=shape, dtype=torch.float32)
+        c = torch.stack([c1, c2], stack_dim)
+
+        shape = list(shape)
+        shape.insert(stack_dim, 2)
+        shape = tuple(shape)
+
+        val = 2 * torch.rand(torch.Size(shape)) - 1
+
+        val_np = c.to_numpy(val)
+        assert isinstance(val_np, np.ndarray)
+        assert (val.numpy() == val_np).all()
+
+        with pytest.raises(AssertionError):
+            c.to_numpy(val + 1)
+
 
 class TestStackComposite:
     def test_stack(self):
@@ -2303,6 +2321,24 @@ def test_clone(self):
         assert cclone[0] is not c[0]
         assert cclone[0] == c[0]
 
+    def test_to_numpy(self):
+        c1 = CompositeSpec(a=BoundedTensorSpec(-1, 1, shape=(1, 3)), shape=(1, 3))
+        c2 = CompositeSpec(
+            a=BoundedTensorSpec(-1, 1, shape=(1, 3)),
+            b=UnboundedDiscreteTensorSpec(shape=(1, 3)),
+            shape=(1, 3),
+        )
+        c = torch.stack([c1, c2], 0)
+        for _ in range(100):
+            r = c.rand()
+            for key, value in c.to_numpy(r).items():
+                spec = c[key]
+                assert (spec.to_numpy(r[key]) == value).all()
+
+        td_fail = TensorDict({"a": torch.rand((2, 1, 3)) + 1}, [2, 1, 3])
+        with pytest.raises(AssertionError):
+            c.to_numpy(td_fail)
+
 
 if __name__ == "__main__":
     args, unknown = argparse.ArgumentParser().parse_known_args()
diff --git a/torchrl/data/tensor_specs.py b/torchrl/data/tensor_specs.py
index 0f36db422be..54ea17fb4e2 100644
--- a/torchrl/data/tensor_specs.py
+++ b/torchrl/data/tensor_specs.py
@@ -668,38 +668,23 @@ def __eq__(self, other):
         # requires unbind to be implemented
         pass
 
-    def to_numpy(self, val: TensorDict, safe: bool = True) -> dict:
-        pass
+    def to_numpy(self, val: torch.Tensor, safe: bool = True) -> dict:
+        if safe:
+            if val.shape[self.dim] != len(self._specs):
+                raise ValueError(
+                    "Size of LazyStackedTensorSpec and val differ along the stacking "
+                    "dimension"
+                )
+            for spec, v in zip(self._specs, torch.unbind(val, dim=self.dim)):
+                spec.assert_is_in(v)
+        return val.detach().cpu().numpy()
 
     def __len__(self):
         pass
 
-    def values(self) -> ValuesView:
-        pass
-
-    def items(self) -> ItemsView:
-        pass
-
-    def keys(
-        self,
-        include_nested: bool = False,
-        leaves_only: bool = False,
-    ) -> KeysView:
-        pass
-
     def project(self, val: TensorDictBase) -> TensorDictBase:
         pass
 
-    def is_in(self, val: Union[dict, TensorDictBase]) -> bool:
-        pass
-
-    def type_check(
-        self,
-        value: Union[torch.Tensor, TensorDictBase],
-        selected_keys: Union[str, Optional[Sequence[str]]] = None,
-    ):
-        pass
-
     def __repr__(self):
         shape_str = "shape=" + str(self.shape)
         space_str = "space=" + str(self._specs[0].space)
@@ -712,12 +697,6 @@ def __repr__(self):
         string = f"{self.__class__.__name__}(\n     {sub_string})"
         return string
 
-    def encode(self, vals: Dict[str, Any]) -> Dict[str, torch.Tensor]:
-        pass
-
-    def __delitem__(self, key):
-        pass
-
     def __iter__(self):
         pass
 
@@ -726,7 +705,7 @@ def __setitem__(self, key, value):
 
     @property
     def device(self) -> DEVICE_TYPING:
-        pass
+        return self._specs[0].device
 
     @property
     def ndim(self):
@@ -2591,7 +2570,15 @@ def __eq__(self, other):
         pass
 
     def to_numpy(self, val: TensorDict, safe: bool = True) -> dict:
-        pass
+        if safe:
+            if val.shape[self.dim] != len(self._specs):
+                raise ValueError(
+                    "Size of LazyStackedCompositeSpec and val differ along the "
+                    "stacking dimension"
+                )
+            for spec, v in zip(self._specs, torch.unbind(val, dim=self.dim)):
+                spec.assert_is_in(v)
+        return {key: self[key].to_numpy(val) for key, val in val.items()}
 
     def __len__(self):
         pass