diff --git a/torchy_baselines/a2c/a2c.py b/torchy_baselines/a2c/a2c.py
index 4c24c7f..6e0a1f2 100644
--- a/torchy_baselines/a2c/a2c.py
+++ b/torchy_baselines/a2c/a2c.py
@@ -7,11 +7,11 @@ import torch.nn.functional as F
 from torchy_baselines.common.utils import explained_variance
 from torchy_baselines.common import logger
 from torchy_baselines.common.type_aliases import GymEnv
+from torchy_baselines.common.callbacks import BaseCallback
 from torchy_baselines.ppo.ppo import PPO
 from torchy_baselines.ppo.policies import PPOPolicy
 
 
-
 class A2C(PPO):
     """
     Advantage Actor Critic (A2C)
@@ -89,14 +89,14 @@ class A2C(PPO):
         if _init_setup_model:
             self._setup_model()
 
-    def _setup_model(self):
+    def _setup_model(self) -> None:
         super(A2C, self)._setup_model()
         if self.use_rms_prop:
             self.policy.optimizer = th.optim.RMSprop(self.policy.parameters(),
                                                      lr=self.learning_rate(1), alpha=0.99,
                                                      eps=self.rms_prop_eps, weight_decay=0)
 
-    def train(self, gradient_steps: int, batch_size=None):
+    def train(self, gradient_steps: int, batch_size: Optional[int] = None) -> None:
         # Update optimizer learning rate
         self._update_learning_rate(self.policy.optimizer)
         # A2C with gradient_steps > 1 does not make sense
@@ -153,9 +153,16 @@ class A2C(PPO):
         if hasattr(self.policy, 'log_std'):
             logger.logkv("std", th.exp(self.policy.log_std).mean().item())
 
-    def learn(self, total_timesteps, callback=None, log_interval=100,
-              eval_env=None, eval_freq=-1, n_eval_episodes=5,
-              tb_log_name="A2C", eval_log_path=None, reset_num_timesteps=True):
+    def learn(self,
+              total_timesteps: int,
+              callback: Optional[BaseCallback] = None,
+              log_interval: int = 100,
+              eval_env: Optional[GymEnv] = None,
+              eval_freq: int = -1,
+              n_eval_episodes: int = 5,
+              tb_log_name: str = "A2C",
+              eval_log_path: Optional[str] = None,
+              reset_num_timesteps: bool = True) -> 'A2C':
 
         return super(A2C, self).learn(total_timesteps=total_timesteps, callback=callback, log_interval=log_interval,
                                       eval_env=eval_env, eval_freq=eval_freq, n_eval_episodes=n_eval_episodes,
diff --git a/torchy_baselines/cem_rl/cem.py b/torchy_baselines/cem_rl/cem.py
index ee4f484..d1d221c 100644
--- a/torchy_baselines/cem_rl/cem.py
+++ b/torchy_baselines/cem_rl/cem.py
@@ -1,3 +1,5 @@
+from typing import Type, Tuple, Optional, List
+
 import numpy as np
 
 
@@ -21,9 +23,16 @@ class CEM(object):
     :param antithetic: (bool) Use a finite difference like method for sampling
         (mu + epsilon, mu - epsilon)
     """
-    def __init__(self, num_params, mu_init=None, sigma_init=1e-3,
-                 pop_size=256, damping_init=1e-3, damping_final=1e-5,
-                 parents=None, elitism=False, antithetic=False):
+    def __init__(self,
+                 num_params: int,
+                 mu_init: Optional[np.ndarray] = None,
+                 sigma_init: float = 1e-3,
+                 pop_size: int = 256,
+                 damping_init: float = 1e-3,
+                 damping_final: float = 1e-5,
+                 parents: Optional[int] = None,
+                 elitism: bool = False,
+                 antithetic: bool = False):
         super(CEM, self).__init__()
 
         self.num_params = num_params
@@ -66,7 +75,7 @@ class CEM(object):
                                  for i in range(1, self.parents + 1)])
         self.weights /= self.weights.sum()
 
-    def ask(self, pop_size):
+    def ask(self, pop_size: int) -> List[np.ndarray]:
         """
         Returns a list of candidates parameters
 
@@ -87,7 +96,7 @@ class CEM(object):
 
         return individuals
 
-    def tell(self, solutions, scores):
+    def tell(self, solutions: List[np.ndarray], scores: List[float]) -> None:
         """
         Updates the distribution
 
@@ -114,7 +123,7 @@ class CEM(object):
         self.elite = solutions[idx_sorted[0]]
         self.elite_score = scores[idx_sorted[0]]
 
-    def get_distrib_params(self):
+    def get_distrib_params(self) -> Tuple[np.ndarray, np.ndarray]:
         """
         Returns the parameters of the distribution:
         the mean and standard deviation.
diff --git a/torchy_baselines/ppo/ppo.py b/torchy_baselines/ppo/ppo.py
index 6f9a483..a7dcf0b 100644
--- a/torchy_baselines/ppo/ppo.py
+++ b/torchy_baselines/ppo/ppo.py
@@ -121,7 +121,7 @@ class PPO(BaseRLModel):
         if _init_setup_model:
             self._setup_model()
 
-    def _setup_model(self):
+    def _setup_model(self) -> None:
         self._setup_learning_rate()
         # TODO: preprocessing: one hot vector for obs discrete
         state_dim = self.observation_space.shape[0]
@@ -284,9 +284,16 @@ class PPO(BaseRLModel):
         if hasattr(self.policy, 'log_std'):
             logger.logkv("std", th.exp(self.policy.log_std).mean().item())
 
-    def learn(self, total_timesteps, callback=None, log_interval=1,
-              eval_env=None, eval_freq=-1, n_eval_episodes=5, tb_log_name="PPO",
-              eval_log_path=None, reset_num_timesteps=True):
+    def learn(self,
+              total_timesteps: int,
+              callback: Optional[BaseCallback] = None,
+              log_interval: int = 1,
+              eval_env: Optional[GymEnv] = None,
+              eval_freq: int = -1,
+              n_eval_episodes: int = 5,
+              tb_log_name: str = "PPO",
+              eval_log_path: Optional[str] = None,
+              reset_num_timesteps: bool = True) -> 'PPO':
 
         episode_num, obs, callback = self._setup_learn(eval_env, callback, eval_freq,
                                                        n_eval_episodes, eval_log_path, reset_num_timesteps)