diff --git a/brax/envs/base.py b/brax/envs/base.py
index 9097ac2f..b9436b2c 100644
--- a/brax/envs/base.py
+++ b/brax/envs/base.py
@@ -16,7 +16,7 @@
 """A brax environment for training and inference."""
 
 import abc
-from typing import Any, Dict, List, Optional, Sequence, Union
+from typing import Any, Dict, List, Mapping, Optional, Sequence, Tuple, Union
 
 from brax import base
 from brax.generalized import pipeline as g_pipeline
@@ -28,13 +28,14 @@
 import jax
 import numpy as np
 
+ObservationSize = Union[int, Mapping[str, Union[Tuple[int, ...], int]]]
 
 @struct.dataclass
 class State(base.Base):
   """Environment state for training and inference."""
 
   pipeline_state: Optional[base.State]
-  obs: jax.Array
+  obs: Union[jax.Array, Mapping[str, jax.Array]]
   reward: jax.Array
   done: jax.Array
   metrics: Dict[str, jax.Array] = struct.field(default_factory=dict)
@@ -54,7 +55,7 @@ def step(self, state: State, action: jax.Array) -> State:
 
   @property
   @abc.abstractmethod
-  def observation_size(self) -> int:
+  def observation_size(self) -> ObservationSize:
     """The size of the observation vector returned in step and reset."""
 
   @property
@@ -139,10 +140,13 @@ def dt(self) -> jax.Array:
     return self.sys.opt.timestep * self._n_frames  # pytype: disable=attribute-error
 
   @property
-  def observation_size(self) -> int:
+  def observation_size(self) -> ObservationSize:
     rng = jax.random.PRNGKey(0)
     reset_state = self.unwrapped.reset(rng)
-    return reset_state.obs.shape[-1]
+    obs = reset_state.obs
+    if isinstance(obs, jax.Array):
+      return obs.shape[-1]
+    return jax.tree_util.tree_map(lambda x: x.shape, obs)
 
   @property
   def action_size(self) -> int:
@@ -176,7 +180,7 @@ def step(self, state: State, action: jax.Array) -> State:
     return self.env.step(state, action)
 
   @property
-  def observation_size(self) -> int:
+  def observation_size(self) -> ObservationSize:
     return self.env.observation_size
 
   @property
diff --git a/brax/envs/fast.py b/brax/envs/fast.py
index f7351b71..f7b312d8 100644
--- a/brax/envs/fast.py
+++ b/brax/envs/fast.py
@@ -28,6 +28,7 @@ def __init__(self, **kwargs):
     self._dt = 0.02
     self._reset_count = 0
     self._step_count = 0
+    self._use_dict_obs = kwargs.get('use_dict_obs', False)
 
   def reset(self, rng: jax.Array) -> State:
     self._reset_count += 1
@@ -39,6 +40,7 @@ def reset(self, rng: jax.Array) -> State:
         contact=None
     )
     obs = jp.zeros(2)
+    obs = {'state': obs} if self._use_dict_obs else obs
     reward, done = jp.array(0.0), jp.array(0.0)
     return State(pipeline_state, obs, reward, done)
 
@@ -53,6 +55,7 @@ def step(self, state: State, action: jax.Array) -> State:
         xd=state.pipeline_state.xd.replace(vel=vel),
     )
     obs = jp.array([pos[0], vel[0]])
+    obs = {'state': obs} if self._use_dict_obs else obs
     reward = pos[0]
 
     return state.replace(pipeline_state=qp, obs=obs, reward=reward)
diff --git a/brax/envs/wrappers/training.py b/brax/envs/wrappers/training.py
index a9b91b70..d4a364d2 100644
--- a/brax/envs/wrappers/training.py
+++ b/brax/envs/wrappers/training.py
@@ -130,7 +130,7 @@ def where_done(x, y):
     pipeline_state = jax.tree.map(
         where_done, state.info['first_pipeline_state'], state.pipeline_state
     )
-    obs = where_done(state.info['first_obs'], state.obs)
+    obs = jax.tree.map(where_done, state.info['first_obs'], state.obs)
     return state.replace(pipeline_state=pipeline_state, obs=obs)
 
 
diff --git a/brax/training/agents/apg/train.py b/brax/training/agents/apg/train.py
index 351a7166..4482b63a 100644
--- a/brax/training/agents/apg/train.py
+++ b/brax/training/agents/apg/train.py
@@ -131,11 +131,15 @@ def train(
   reset_fn = jax.jit(jax.vmap(env.reset))
   step_fn = jax.jit(jax.vmap(env.step))
 
+  obs_size = env.observation_size
+  if isinstance(obs_size, Dict):
+    raise NotImplementedError("Dictionary observations not implemented in APG")
+
   normalize = lambda x, y: x
   if normalize_observations:
     normalize = running_statistics.normalize
   apg_network = network_factory(
-      env.observation_size,
+      obs_size,
       env.action_size,
       preprocess_observations_fn=normalize)
   make_policy = apg_networks.make_inference_fn(apg_network)
diff --git a/brax/training/agents/ars/train.py b/brax/training/agents/ars/train.py
index 70d8a003..77395e25 100644
--- a/brax/training/agents/ars/train.py
+++ b/brax/training/agents/ars/train.py
@@ -119,6 +119,8 @@ def train(
     )
 
   obs_size = env.observation_size
+  if isinstance(obs_size, Dict):
+    raise NotImplementedError("Dictionary observations not implemented in ARS")
 
   normalize_fn = lambda x, y: x
   if normalize_observations:
diff --git a/brax/training/agents/es/train.py b/brax/training/agents/es/train.py
index 7351f3dc..c3b20ba5 100644
--- a/brax/training/agents/es/train.py
+++ b/brax/training/agents/es/train.py
@@ -146,7 +146,9 @@ def train(
     )
 
   obs_size = env.observation_size
-
+  if isinstance(obs_size, Dict):
+    raise NotImplementedError("Dictionary observations not implemented in ES")
+  
   normalize_fn = lambda x, y: x
   if normalize_observations:
     normalize_fn = running_statistics.normalize
diff --git a/brax/training/agents/ppo/losses.py b/brax/training/agents/ppo/losses.py
index c477739c..202aa796 100644
--- a/brax/training/agents/ppo/losses.py
+++ b/brax/training/agents/ppo/losses.py
@@ -135,9 +135,9 @@ def compute_ppo_loss(
                                data.observation)
 
   baseline = value_apply(normalizer_params, params.value, data.observation)
-
+  terminal_obs = jax.tree_util.tree_map(lambda x: x[-1], data.next_observation)
   bootstrap_value = value_apply(normalizer_params, params.value,
-                                data.next_observation[-1])
+                                terminal_obs)
 
   rewards = data.reward * reward_scaling
   truncation = data.extras['state_extras']['truncation']
diff --git a/brax/training/agents/ppo/train.py b/brax/training/agents/ppo/train.py
index a7932fc6..137db298 100644
--- a/brax/training/agents/ppo/train.py
+++ b/brax/training/agents/ppo/train.py
@@ -233,12 +233,16 @@ def train(
   key_envs = jnp.reshape(key_envs,
                          (local_devices_to_use, -1) + key_envs.shape[1:])
   env_state = reset_fn(key_envs)
+  ndarray_obs = isinstance(env_state.obs, jnp.ndarray) # Check whether observations are in dictionary form.
+
+  # Discard the batch axes over devices and envs.
+  obs_shape = jax.tree_util.tree_map(lambda x: x.shape[2:], env_state.obs)
 
   normalize = lambda x, y: x
   if normalize_observations:
     normalize = running_statistics.normalize
   ppo_network = network_factory(
-      env_state.obs.shape[-1],
+      obs_shape,
       env.action_size,
       preprocess_observations_fn=normalize)
   make_policy = ppo_networks.make_inference_fn(ppo_network)
@@ -332,7 +336,7 @@ def f(carry, unused_t):
     # Update normalization params and normalize observations.
     normalizer_params = running_statistics.update(
         training_state.normalizer_params,
-        data.observation,
+        data.observation if ndarray_obs else data.observation['state'],
         pmap_axis_name=_PMAP_AXIS_NAME)
 
     (optimizer_state, params, _), metrics = jax.lax.scan(
@@ -389,11 +393,12 @@ def training_epoch_with_timing(
       value=ppo_network.value_network.init(key_value),
   )
 
+  obs_shape = env_state.obs.shape if ndarray_obs else env_state.obs['state'].shape
   training_state = TrainingState(  # pytype: disable=wrong-arg-types  # jax-ndarray
       optimizer_state=optimizer.init(init_params),  # pytype: disable=wrong-arg-types  # numpy-scalars
       params=init_params,
       normalizer_params=running_statistics.init_state(
-          specs.Array(env_state.obs.shape[-1:], jnp.dtype('float32'))),
+          specs.Array(obs_shape[-1:], jnp.dtype('float32'))),
       env_steps=0)
 
   if (
diff --git a/brax/training/agents/ppo/train_test.py b/brax/training/agents/ppo/train_test.py
index 408bd4f4..bd54af01 100644
--- a/brax/training/agents/ppo/train_test.py
+++ b/brax/training/agents/ppo/train_test.py
@@ -27,10 +27,10 @@
 class PPOTest(parameterized.TestCase):
   """Tests for PPO module."""
 
-
-  def testTrain(self):
+  @parameterized.parameters(True, False)
+  def testTrain(self, use_dict_obs):
     """Test PPO with a simple env."""
-    fast = envs.get_environment('fast')
+    fast = envs.get_environment('fast', use_dict_obs=use_dict_obs)
     _, _, metrics = ppo.train(
         fast,
         num_timesteps=2**15,
diff --git a/brax/training/agents/sac/train.py b/brax/training/agents/sac/train.py
index 64053a81..08da8bbf 100644
--- a/brax/training/agents/sac/train.py
+++ b/brax/training/agents/sac/train.py
@@ -191,6 +191,9 @@ def train(
     )
 
   obs_size = env.observation_size
+  if isinstance(obs_size, Dict):
+    raise NotImplementedError("Dictionary observations not implemented in SAC")
+
   action_size = env.action_size
 
   normalize_fn = lambda x, y: x
diff --git a/brax/training/networks.py b/brax/training/networks.py
index 23e041aa..10a429c0 100644
--- a/brax/training/networks.py
+++ b/brax/training/networks.py
@@ -15,7 +15,7 @@
 """Network definitions."""
 
 import dataclasses
-from typing import Any, Callable, Sequence, Tuple
+from typing import Any, Callable, Mapping, Sequence, Tuple
 import warnings
 
 from brax.training import types
@@ -82,10 +82,13 @@ def __call__(self, data: jnp.ndarray):
         hidden = self.activation(hidden)
     return hidden
 
+def get_obs_state_size(obs_size: types.ObservationSize) -> int:
+    obs_size = obs_size['state'] if isinstance(obs_size, Mapping) else obs_size
+    return jax.tree_util.tree_flatten(obs_size)[0][-1] # Size can be tuple or int.
 
 def make_policy_network(
     param_size: int,
-    obs_size: int,
+    obs_size: types.ObservationSize,
     preprocess_observations_fn: types.PreprocessObservationFn = types
     .identity_observation_preprocessor,
     hidden_layer_sizes: Sequence[int] = (256, 256),
@@ -100,30 +103,36 @@ def make_policy_network(
       layer_norm=layer_norm)
 
   def apply(processor_params, policy_params, obs):
+    obs = (obs if isinstance(obs, jnp.ndarray)
+      else obs['state']) # state-only in the case of dict obs.
     obs = preprocess_observations_fn(obs, processor_params)
     return policy_module.apply(policy_params, obs)
 
+  obs_size = get_obs_state_size(obs_size)
   dummy_obs = jnp.zeros((1, obs_size))
   return FeedForwardNetwork(
       init=lambda key: policy_module.init(key, dummy_obs), apply=apply)
 
 
 def make_value_network(
-    obs_size: int,
+    obs_size: types.ObservationSize,
     preprocess_observations_fn: types.PreprocessObservationFn = types
     .identity_observation_preprocessor,
     hidden_layer_sizes: Sequence[int] = (256, 256),
     activation: ActivationFn = linen.relu) -> FeedForwardNetwork:
-  """Creates a policy network."""
+  """Creates a value network."""
   value_module = MLP(
       layer_sizes=list(hidden_layer_sizes) + [1],
       activation=activation,
       kernel_init=jax.nn.initializers.lecun_uniform())
 
-  def apply(processor_params, policy_params, obs):
+  def apply(processor_params, value_params, obs):
+    obs = (obs if isinstance(obs, jnp.ndarray)
+      else obs['state']) # state-only in the case of dict obs.
     obs = preprocess_observations_fn(obs, processor_params)
-    return jnp.squeeze(value_module.apply(policy_params, obs), axis=-1)
+    return jnp.squeeze(value_module.apply(value_params, obs), axis=-1)
 
+  obs_size = get_obs_state_size(obs_size)
   dummy_obs = jnp.zeros((1, obs_size))
   return FeedForwardNetwork(
       init=lambda key: value_module.init(key, dummy_obs), apply=apply)
diff --git a/brax/training/types.py b/brax/training/types.py
index 63fbf129..61839cf6 100644
--- a/brax/training/types.py
+++ b/brax/training/types.py
@@ -14,7 +14,7 @@
 
 """Brax training types."""
 
-from typing import Any, Mapping, NamedTuple, Tuple, TypeVar
+from typing import Any, Mapping, NamedTuple, Tuple, TypeVar, Union
 
 from brax.training.acme.types import NestedArray
 import jax.numpy as jnp
@@ -30,7 +30,8 @@
 Params = Any
 PRNGKey = jnp.ndarray
 Metrics = Mapping[str, jnp.ndarray]
-Observation = jnp.ndarray
+Observation = Union[jnp.ndarray, Mapping[str, jnp.ndarray]]
+ObservationSize = Union[int, Mapping[str, Union[Tuple[int, ...], int]]]
 Action = jnp.ndarray
 Extra = Mapping[str, Any]
 PolicyParams = Any
@@ -79,7 +80,7 @@ class NetworkFactory(Protocol[NetworkType]):
 
   def __call__(
       self,
-      observation_size: int,
+      observation_size: ObservationSize,
       action_size: int,
       preprocess_observations_fn:
       PreprocessObservationFn = identity_observation_preprocessor