diff --git a/docs/misc/changelog.rst b/docs/misc/changelog.rst
index 0ca6fa9..a65ec1d 100644
--- a/docs/misc/changelog.rst
+++ b/docs/misc/changelog.rst
@@ -21,6 +21,7 @@ New Features:
   when ``psutil`` is available
 - Saving models now automatically creates the necessary folders and raises appropriate warnings (@PartiallyTyped)
 - Refactored opening paths for saving and loading to use strings, pathlib or io.BufferedIOBase (@PartiallyTyped)
+- Introduced ``BaseModel`` abstract parent for ``BasePolicy``, which critics inherit from.
 
 Bug Fixes:
 ^^^^^^^^^^
diff --git a/stable_baselines3/common/base_class.py b/stable_baselines3/common/base_class.py
index f66220e..4e51367 100644
--- a/stable_baselines3/common/base_class.py
+++ b/stable_baselines3/common/base_class.py
@@ -320,8 +320,8 @@ class BaseAlgorithm(ABC):
                              f"Stored kwargs: {data['policy_kwargs']}, specified kwargs: {kwargs['policy_kwargs']}")
 
         # check if observation space and action space are part of the saved parameters
-        if ("observation_space" not in data or "action_space" not in data) and "env" not in data:
-            raise ValueError("The observation_space and action_space was not given, can't verify new environments")
+        if "observation_space" not in data or "action_space" not in data:
+            raise KeyError("The observation_space and action_space were not given, can't verify new environments")
         # check if given env is valid
         if env is not None:
             check_for_correct_spaces(env, data["observation_space"], data["action_space"])
@@ -425,8 +425,10 @@ class BaseAlgorithm(ABC):
         :return: (Tuple[int, BaseCallback])
         """
         self.start_time = time.time()
-        self.ep_info_buffer = deque(maxlen=100)
-        self.ep_success_buffer = deque(maxlen=100)
+        if self.ep_info_buffer is None or reset_num_timesteps:
+            # Initialize buffers if they don't exist, or reinitialize if resetting counters
+            self.ep_info_buffer = deque(maxlen=100)
+            self.ep_success_buffer = deque(maxlen=100)
 
         if self.action_noise is not None:
             self.action_noise.reset()
diff --git a/stable_baselines3/common/distributions.py b/stable_baselines3/common/distributions.py
index 6fa70f8..1a20928 100644
--- a/stable_baselines3/common/distributions.py
+++ b/stable_baselines3/common/distributions.py
@@ -17,6 +17,20 @@ class Distribution(ABC):
     def __init__(self):
         super(Distribution, self).__init__()
 
+    @abstractmethod
+    def proba_distribution_net(self, *args, **kwargs):
+        """Create the layers and parameters that represent the distribution.
+
+        Subclasses must define this, but the arguments and return type vary between
+        concrete classes."""
+
+    @abstractmethod
+    def proba_distribution(self, *args, **kwargs) -> 'Distribution':
+        """Set parameters of the distribution.
+
+        :return: (Distribution) self
+        """
+
     @abstractmethod
     def log_prob(self, x: th.Tensor) -> th.Tensor:
         """
diff --git a/stable_baselines3/common/policies.py b/stable_baselines3/common/policies.py
index 08b0ab2..bc387c2 100644
--- a/stable_baselines3/common/policies.py
+++ b/stable_baselines3/common/policies.py
@@ -21,9 +21,12 @@ from stable_baselines3.common.distributions import (make_proba_distribution, Dis
                                                     StateDependentNoiseDistribution)
 
 
-class BasePolicy(nn.Module, ABC):
+class BaseModel(nn.Module, ABC):
     """
-    The base policy object
+    The base model object: makes predictions in response to observations.
+
+    In the case of policies, the prediction is an action. In the case of critics, it is the
+    estimated value of the observation.
 
     :param observation_space: (gym.spaces.Space) The observation space of the environment
     :param action_space: (gym.spaces.Space) The action space of the environment
@@ -39,8 +42,6 @@ class BasePolicy(nn.Module, ABC):
         ``th.optim.Adam`` by default
     :param optimizer_kwargs: (Optional[Dict[str, Any]]) Additional keyword arguments,
         excluding the learning rate, to pass to the optimizer
-    :param squash_output: (bool) For continuous actions, whether the output is squashed
-        or not using a ``tanh()`` function.
     """
 
     def __init__(self,
@@ -52,9 +53,8 @@ class BasePolicy(nn.Module, ABC):
                  features_extractor: Optional[nn.Module] = None,
                  normalize_images: bool = True,
                  optimizer_class: Type[th.optim.Optimizer] = th.optim.Adam,
-                 optimizer_kwargs: Optional[Dict[str, Any]] = None,
-                 squash_output: bool = False):
-        super(BasePolicy, self).__init__()
+                 optimizer_kwargs: Optional[Dict[str, Any]] = None):
+        super(BaseModel, self).__init__()
 
         if optimizer_kwargs is None:
             optimizer_kwargs = {}
@@ -67,7 +67,6 @@ class BasePolicy(nn.Module, ABC):
         self.device = get_device(device)
         self.features_extractor = features_extractor
         self.normalize_images = normalize_images
-        self._squash_output = squash_output
 
         self.optimizer_class = optimizer_class
         self.optimizer_kwargs = optimizer_kwargs
@@ -76,6 +75,10 @@ class BasePolicy(nn.Module, ABC):
         self.features_extractor_class = features_extractor_class
         self.features_extractor_kwargs = features_extractor_kwargs
 
+    @abstractmethod
+    def forward(self, *args, **kwargs):
+        del args, kwargs
+
     def extract_features(self, obs: th.Tensor) -> th.Tensor:
         """
         Preprocess the observation if needed and extract features.
@@ -87,9 +90,88 @@ class BasePolicy(nn.Module, ABC):
         preprocessed_obs = preprocess_obs(obs, self.observation_space, normalize_images=self.normalize_images)
         return self.features_extractor(preprocessed_obs)
 
+    def _get_data(self) -> Dict[str, Any]:
+        """
+        Get data that need to be saved in order to re-create the model.
+        This corresponds to the arguments of the constructor.
+
+        :return: (Dict[str, Any])
+        """
+        return dict(
+            observation_space=self.observation_space,
+            action_space=self.action_space,
+            # Passed to the constructor by child class
+            # squash_output=self.squash_output,
+            # features_extractor=self.features_extractor
+            normalize_images=self.normalize_images,
+        )
+
+    def save(self, path: str) -> None:
+        """
+        Save model to a given location.
+
+        :param path: (str)
+        """
+        th.save({'state_dict': self.state_dict(), 'data': self._get_data()}, path)
+
+    @classmethod
+    def load(cls, path: str, device: Union[th.device, str] = 'auto') -> 'BaseModel':
+        """
+        Load model from path.
+
+        :param path: (str)
+        :param device: (Union[th.device, str]) Device on which the policy should be loaded.
+        :return: (BasePolicy)
+        """
+        device = get_device(device)
+        saved_variables = th.load(path, map_location=device)
+        # Create policy object
+        model = cls(**saved_variables['data'])
+        # Load weights
+        model.load_state_dict(saved_variables['state_dict'])
+        model.to(device)
+        return model
+
+    def load_from_vector(self, vector: np.ndarray):
+        """
+        Load parameters from a 1D vector.
+
+        :param vector: (np.ndarray)
+        """
+        th.nn.utils.vector_to_parameters(th.FloatTensor(vector).to(self.device), self.parameters())
+
+    def parameters_to_vector(self) -> np.ndarray:
+        """
+        Convert the parameters to a 1D vector.
+
+        :return: (np.ndarray)
+        """
+        return th.nn.utils.parameters_to_vector(self.parameters()).detach().cpu().numpy()
+
+
+class BasePolicy(BaseModel):
+    """The base policy object.
+
+    Parameters are mostly the same as `BaseModel`; additions are documented below.
+
+    :param args: positional arguments passed through to `BaseModel`.
+    :param kwargs: keyword arguments passed through to `BaseModel`.
+    :param squash_output: (bool) For continuous actions, whether the output is squashed
+        or not using a ``tanh()`` function.
+    """
+    def __init__(self, *args, squash_output: bool = False, **kwargs):
+        super(BasePolicy, self).__init__(*args, **kwargs)
+        self._squash_output = squash_output
+
+    @staticmethod
+    def _dummy_schedule(progress_remaining: float) -> float:
+        """ (float) Useful for pickling policy."""
+        del progress_remaining
+        return 0.0
+
     @property
     def squash_output(self) -> bool:
-        """ (bool) Getter for squash_output."""
+        """(bool) Getter for squash_output."""
         return self._squash_output
 
     @staticmethod
@@ -101,16 +183,7 @@ class BasePolicy(nn.Module, ABC):
             nn.init.orthogonal_(module.weight, gain=gain)
             module.bias.data.fill_(0.0)
 
-    @staticmethod
-    def _dummy_schedule(progress_remaining: float) -> float:
-        """ (float) Useful for pickling policy."""
-        del progress_remaining
-        return 0.0
-
     @abstractmethod
-    def forward(self, *args, **kwargs):
-        del args, kwargs
-
     def _predict(self, observation: th.Tensor, deterministic: bool = False) -> th.Tensor:
         """
         Get the action according to the policy for a given observation.
@@ -122,7 +195,6 @@ class BasePolicy(nn.Module, ABC):
         :param deterministic: (bool) Whether to use stochastic or deterministic actions
         :return: (th.Tensor) Taken action according to the policy
         """
-        raise NotImplementedError()
 
     def predict(self,
                 observation: np.ndarray,
@@ -140,6 +212,7 @@ class BasePolicy(nn.Module, ABC):
         :return: (Tuple[np.ndarray, Optional[np.ndarray]]) the model's action and the next state
             (used in recurrent policies)
         """
+        # TODO (GH/1): add support for RNN policies
         # if state is None:
         #     state = self.initial_state
         # if mask is None:
@@ -204,64 +277,6 @@ class BasePolicy(nn.Module, ABC):
         low, high = self.action_space.low, self.action_space.high
         return low + (0.5 * (scaled_action + 1.0) * (high - low))
 
-    def _get_data(self) -> Dict[str, Any]:
-        """
-        Get data that need to be saved in order to re-create the policy.
-        This corresponds to the arguments of the constructor.
-
-        :return: (Dict[str, Any])
-        """
-        return dict(
-            observation_space=self.observation_space,
-            action_space=self.action_space,
-            # Passed to the constructor by child class
-            # squash_output=self.squash_output,
-            # features_extractor=self.features_extractor
-            normalize_images=self.normalize_images,
-        )
-
-    def save(self, path: str) -> None:
-        """
-        Save policy to a given location.
-
-        :param path: (str)
-        """
-        th.save({'state_dict': self.state_dict(), 'data': self._get_data()}, path)
-
-    @classmethod
-    def load(cls, path: str, device: Union[th.device, str] = 'auto') -> 'BasePolicy':
-        """
-        Load policy from path.
-
-        :param path: (str)
-        :param device: (Union[th.device, str]) Device on which the policy should be loaded.
-        :return: (BasePolicy)
-        """
-        device = get_device(device)
-        saved_variables = th.load(path, map_location=device)
-        # Create policy object
-        model = cls(**saved_variables['data'])
-        # Load weights
-        model.load_state_dict(saved_variables['state_dict'])
-        model.to(device)
-        return model
-
-    def load_from_vector(self, vector: np.ndarray):
-        """
-        Load parameters from a 1D vector.
-
-        :param vector: (np.ndarray)
-        """
-        th.nn.utils.vector_to_parameters(th.FloatTensor(vector).to(self.device), self.parameters())
-
-    def parameters_to_vector(self) -> np.ndarray:
-        """
-        Convert the parameters to a 1D vector.
-
-        :return: (np.ndarray)
-        """
-        return th.nn.utils.parameters_to_vector(self.parameters()).detach().cpu().numpy()
-
 
 class ActorCriticPolicy(BasePolicy):
     """
@@ -438,6 +453,8 @@ class ActorCriticPolicy(BasePolicy):
             self.action_net = self.action_dist.proba_distribution_net(latent_dim=latent_dim_pi)
         elif isinstance(self.action_dist, BernoulliDistribution):
             self.action_net = self.action_dist.proba_distribution_net(latent_dim=latent_dim_pi)
+        else:
+            raise NotImplementedError(f"Unsupported distribution '{self.action_dist}'.")
 
         self.value_net = nn.Linear(self.mlp_extractor.latent_dim_vf, 1)
         # Init weights: use orthogonal initialization
@@ -626,7 +643,7 @@ class ActorCriticCnnPolicy(ActorCriticPolicy):
                                                    optimizer_kwargs)
 
 
-class ContinuousCritic(BasePolicy):
+class ContinuousCritic(BaseModel):
     """
     Critic network(s) for DDPG/SAC/TD3.
     It represents the action-state value function (Q-value function).