diff --git a/lerobot/common/datasets/factory.py b/lerobot/common/datasets/factory.py
index 4ae161f6..10106fe9 100644
--- a/lerobot/common/datasets/factory.py
+++ b/lerobot/common/datasets/factory.py
@@ -86,11 +86,10 @@ def make_dataset(
             ]
         )
 
-    delta_timestamps = cfg.policy.get("delta_timestamps")
-    if delta_timestamps is not None:
-        for key in delta_timestamps:
-            if isinstance(delta_timestamps[key], str):
-                delta_timestamps[key] = eval(delta_timestamps[key])
+    delta_timestamps = cfg.policy.delta_timestamps
+    for key in delta_timestamps:
+        if isinstance(delta_timestamps[key], str):
+            delta_timestamps[key] = eval(delta_timestamps[key])
 
     dataset = clsfunc(
         dataset_id=cfg.dataset_id,
diff --git a/lerobot/common/policies/act/configuration_act.py b/lerobot/common/policies/act/configuration_act.py
new file mode 100644
index 00000000..a3dc1590
--- /dev/null
+++ b/lerobot/common/policies/act/configuration_act.py
@@ -0,0 +1,68 @@
+from dataclasses import dataclass
+
+
+@dataclass
+class ActConfig:
+    """
+    TODO(now): Document all variables
+    TODO(now): Pick sensible defaults for a use case?
+    """
+
+    # Environment.
+    state_dim: int
+    action_dim: int
+
+    # Inputs / output structure.
+    n_obs_steps: int
+    camera_names: list[str]
+    chunk_size: int
+    n_action_steps: int
+
+    # Vision preprocessing.
+    image_normalization_mean: tuple[float, float, float]
+    image_normalization_std: tuple[float, float, float]
+
+    # Architecture.
+    # Vision backbone.
+    vision_backbone: str
+    use_pretrained_backbone: bool
+    replace_final_stride_with_dilation: int
+    # Transformer layers.
+    pre_norm: bool
+    d_model: int
+    n_heads: int
+    dim_feedforward: int
+    feedforward_activation: str
+    n_encoder_layers: int
+    n_decoder_layers: int
+    # VAE.
+    use_vae: bool
+    latent_dim: int
+    n_vae_encoder_layers: int
+
+    # Inference.
+    use_temporal_aggregation: bool
+
+    # Training and loss computation.
+    dropout: float
+    kl_weight: float
+
+    # ---
+    # TODO(alexander-soare): Remove these from the policy config.
+    batch_size: int
+    lr: float
+    lr_backbone: float
+    weight_decay: float
+    grad_clip_norm: float
+    utd: int
+
+    def __post_init__(self):
+        """Input validation."""
+        if not self.vision_backbone.startswith("resnet"):
+            raise ValueError("`vision_backbone` must be one of the ResNet variants.")
+        if self.use_temporal_aggregation:
+            raise NotImplementedError("Temporal aggregation is not yet implemented.")
+        if self.n_action_steps > self.chunk_size:
+            raise ValueError(
+                "The chunk size is the upper bound for the number of action steps per model invocation."
+            )
diff --git a/lerobot/common/policies/act/modeling_act.py b/lerobot/common/policies/act/modeling_act.py
new file mode 100644
index 00000000..769c9470
--- /dev/null
+++ b/lerobot/common/policies/act/modeling_act.py
@@ -0,0 +1,626 @@
+"""Action Chunking Transformer Policy
+
+As per Learning Fine-Grained Bimanual Manipulation with Low-Cost Hardware (https://arxiv.org/abs/2304.13705).
+The majority of changes here involve removing unused code, unifying naming, and adding helpful comments.
+"""
+
+import math
+import time
+from collections import deque
+from itertools import chain
+from typing import Callable
+
+import einops
+import numpy as np
+import torch
+import torch.nn.functional as F  # noqa: N812
+import torchvision
+import torchvision.transforms as transforms
+from torch import Tensor, nn
+from torchvision.models._utils import IntermediateLayerGetter
+from torchvision.ops.misc import FrozenBatchNorm2d
+
+from lerobot.common.policies.act.configuration_act import ActConfig
+
+
+class ActPolicy(nn.Module):
+    """
+    Action Chunking Transformer Policy as per Learning Fine-Grained Bimanual Manipulation with Low-Cost
+    Hardware (paper: https://arxiv.org/abs/2304.13705, code: https://github.com/tonyzhaozh/act)
+
+    Note: In this code we use the terms `vae_encoder`, 'encoder', `decoder`. The meanings are as follows.
+        - The `vae_encoder` is, as per the literature around variational auto-encoders (VAE), the part of the
+          model that encodes the target data (a sequence of actions), and the condition (the robot
+          joint-space).
+        - A transformer with an `encoder` (not the VAE encoder) and `decoder` (not the VAE decoder) with
+          cross-attention is used as the VAE decoder. For these terms, we drop the `vae_` prefix because we
+          have an option to train this model without the variational objective (in which case we drop the
+          `vae_encoder` altogether, and nothing about this model has anything to do with a VAE).
+
+                                 Transformer
+                                 Used alone for inference
+                                 (acts as VAE decoder
+                                  during training)
+                                ┌───────────────────────┐
+                                │             Outputs   │
+                                │                ▲      │
+                                │     ┌─────►┌───────┐  │
+                   ┌──────┐     │     │      │Transf.│  │
+                   │      │     │     ├─────►│decoder│  │
+              ┌────┴────┐ │     │     │      │       │  │
+              │         │ │     │ ┌───┴───┬─►│       │  │
+              │ VAE     │ │     │ │       │  └───────┘  │
+              │ encoder │ │     │ │Transf.│             │
+              │         │ │     │ │encoder│             │
+              └───▲─────┘ │     │ │       │             │
+                  │       │     │ └───▲───┘             │
+                  │       │     │     │                 │
+                inputs    └─────┼─────┘                 │
+                                │                       │
+                                └───────────────────────┘
+    """
+
+    name = "act"
+    _multiple_obs_steps_not_handled_msg = "ActPolicy does not handle multiple observation steps."
+
+    def __init__(self, cfg: ActConfig):
+        """
+        TODO(alexander-soare): Add documentation for all parameters once we have model configs established.
+        """
+        super().__init__()
+        if getattr(cfg, "n_obs_steps", 1) != 1:
+            raise ValueError(self._multiple_obs_steps_not_handled_msg)
+        self.cfg = cfg
+
+        # BERT style VAE encoder with input [cls, *joint_space_configuration, *action_sequence].
+        # The cls token forms parameters of the latent's distribution (like this [*means, *log_variances]).
+        if self.cfg.use_vae:
+            self.vae_encoder = _TransformerEncoder(cfg)
+            self.vae_encoder_cls_embed = nn.Embedding(1, cfg.d_model)
+            # Projection layer for joint-space configuration to hidden dimension.
+            self.vae_encoder_robot_state_input_proj = nn.Linear(cfg.state_dim, cfg.d_model)
+            # Projection layer for action (joint-space target) to hidden dimension.
+            self.vae_encoder_action_input_proj = nn.Linear(cfg.state_dim, cfg.d_model)
+            self.latent_dim = cfg.latent_dim
+            # Projection layer from the VAE encoder's output to the latent distribution's parameter space.
+            self.vae_encoder_latent_output_proj = nn.Linear(cfg.d_model, self.latent_dim * 2)
+            # Fixed sinusoidal positional embedding the whole input to the VAE encoder. Unsqueeze for batch
+            # dimension.
+            self.register_buffer(
+                "vae_encoder_pos_enc",
+                _create_sinusoidal_position_embedding(1 + 1 + cfg.chunk_size, cfg.d_model).unsqueeze(0),
+            )
+
+        # Backbone for image feature extraction.
+        self.image_normalizer = transforms.Normalize(
+            mean=cfg.image_normalization_mean, std=cfg.image_normalization_std
+        )
+        backbone_model = getattr(torchvision.models, cfg.vision_backbone)(
+            replace_stride_with_dilation=[False, False, cfg.replace_final_stride_with_dilation],
+            pretrained=cfg.use_pretrained_backbone,
+            norm_layer=FrozenBatchNorm2d,
+        )
+        # Note: The assumption here is that we are using a ResNet model (and hence layer4 is the final feature
+        # map).
+        # Note: The forward method of this returns a dict: {"feature_map": output}.
+        self.backbone = IntermediateLayerGetter(backbone_model, return_layers={"layer4": "feature_map"})
+
+        # Transformer (acts as VAE decoder when training with the variational objective).
+        self.encoder = _TransformerEncoder(cfg)
+        self.decoder = _TransformerDecoder(cfg)
+
+        # Transformer encoder input projections. The tokens will be structured like
+        # [latent, robot_state, image_feature_map_pixels].
+        self.encoder_robot_state_input_proj = nn.Linear(cfg.state_dim, cfg.d_model)
+        self.encoder_latent_input_proj = nn.Linear(self.latent_dim, cfg.d_model)
+        self.encoder_img_feat_input_proj = nn.Conv2d(
+            backbone_model.fc.in_features, cfg.d_model, kernel_size=1
+        )
+        # Transformer encoder positional embeddings.
+        self.encoder_robot_and_latent_pos_embed = nn.Embedding(2, cfg.d_model)
+        self.encoder_cam_feat_pos_embed = _SinusoidalPositionEmbedding2D(cfg.d_model // 2)
+
+        # Transformer decoder.
+        # Learnable positional embedding for the transformer's decoder (in the style of DETR object queries).
+        self.decoder_pos_embed = nn.Embedding(cfg.chunk_size, cfg.d_model)
+
+        # Final action regression head on the output of the transformer's decoder.
+        self.action_head = nn.Linear(cfg.d_model, cfg.action_dim)
+
+        self._reset_parameters()
+        self._create_optimizer()
+
+    def _create_optimizer(self):
+        optimizer_params_dicts = [
+            {
+                "params": [
+                    p for n, p in self.named_parameters() if not n.startswith("backbone") and p.requires_grad
+                ]
+            },
+            {
+                "params": [
+                    p for n, p in self.named_parameters() if n.startswith("backbone") and p.requires_grad
+                ],
+                "lr": self.cfg.lr_backbone,
+            },
+        ]
+        self.optimizer = torch.optim.AdamW(
+            optimizer_params_dicts, lr=self.cfg.lr, weight_decay=self.cfg.weight_decay
+        )
+
+    def _reset_parameters(self):
+        """Xavier-uniform initialization of the transformer parameters as in the original code."""
+        for p in chain(self.encoder.parameters(), self.decoder.parameters()):
+            if p.dim() > 1:
+                nn.init.xavier_uniform_(p)
+
+    def reset(self):
+        """This should be called whenever the environment is reset."""
+        if self.cfg.n_action_steps is not None:
+            self._action_queue = deque([], maxlen=self.cfg.n_action_steps)
+
+    @torch.no_grad
+    def select_action(self, batch: dict[str, Tensor], **_) -> Tensor:
+        """
+        This method wraps `select_actions` in order to return one action at a time for execution in the
+        environment. It works by managing the actions in a queue and only calling `select_actions` when the
+        queue is empty.
+        """
+        if len(self._action_queue) == 0:
+            # `select_actions` returns a (batch_size, n_action_steps, *) tensor, but the queue effectively has
+            # shape (n_action_steps, batch_size, *), hence the transpose.
+            self._action_queue.extend(self.select_actions(batch).transpose(0, 1))
+        return self._action_queue.popleft()
+
+    @torch.no_grad
+    def select_actions(self, batch: dict[str, Tensor]) -> Tensor:
+        """Use the action chunking transformer to generate a sequence of actions."""
+        self.eval()
+        self._preprocess_batch(batch, add_obs_steps_dim=True)
+
+        action = self.forward(batch, return_loss=False)
+
+        return action[: self.cfg.n_action_steps]
+
+    def __call__(self, *args, **kwargs) -> dict:
+        # TODO(alexander-soare): Temporary bridge until we know what to do about the `update` method.
+        return self.update(*args, **kwargs)
+
+    def _preprocess_batch(
+        self, batch: dict[str, Tensor], add_obs_steps_dim: bool = False
+    ) -> dict[str, Tensor]:
+        """
+        This function expects `batch` to have (at least):
+        {
+            "observation.state": (B, 1, J) OR (B, J) tensor of robot states (joint configuration).
+            "observation.images.top": (B, 1, C, H, W) OR (B, C, H, W) tensor of images.
+        }
+        """
+        if add_obs_steps_dim:
+            # Add a dimension for the observations steps. Since n_obs_steps > 1 is not supported right now,
+            # this just amounts to an unsqueeze.
+            for k in batch:
+                if k.startswith("observation."):
+                    batch[k] = batch[k].unsqueeze(1)
+
+        if batch["observation.state"].shape[1] != 1:
+            raise ValueError(self._multiple_obs_steps_not_handled_msg)
+        batch["observation.state"] = batch["observation.state"].squeeze(1)
+        # TODO(alexander-soare): generalize this to multiple images.
+        assert (
+            sum(k.startswith("observation.images.") and not k.endswith("is_pad") for k in batch) == 1
+        ), "ACT only handles one image for now."
+        # Note: no squeeze is required for "observation.images.top" because then we'd have to unsqueeze to get
+        # the image index dimension.
+
+    def update(self, batch, **_) -> dict:
+        """Run the model in train mode, compute the loss, and do an optimization step."""
+        start_time = time.time()
+        self._preprocess_batch(batch)
+
+        self.train()
+
+        num_slices = self.cfg.batch_size
+        batch_size = self.cfg.chunk_size * num_slices
+
+        assert batch_size % self.cfg.chunk_size == 0
+        assert batch_size % num_slices == 0
+
+        loss = self.forward(batch, return_loss=True)["loss"]
+        loss.backward()
+
+        grad_norm = torch.nn.utils.clip_grad_norm_(
+            self.parameters(),
+            self.cfg.grad_clip_norm,
+            error_if_nonfinite=False,
+        )
+
+        self.optimizer.step()
+        self.optimizer.zero_grad()
+
+        info = {
+            "loss": loss.item(),
+            "grad_norm": float(grad_norm),
+            "lr": self.cfg.lr,
+            "update_s": time.time() - start_time,
+        }
+
+        return info
+
+    def forward(self, batch: dict[str, Tensor], return_loss: bool = False) -> dict | Tensor:
+        """A forward pass through the DNN part of this policy with optional loss computation."""
+        images = self.image_normalizer(batch["observation.images.top"])
+
+        if return_loss:  # training time
+            actions_hat, (mu_hat, log_sigma_x2_hat) = self._forward(
+                batch["observation.state"], images, batch["action"]
+            )
+
+            l1_loss = (
+                F.l1_loss(batch["action"], actions_hat, reduction="none")
+                * ~batch["action_is_pad"].unsqueeze(-1)
+            ).mean()
+
+            loss_dict = {}
+            loss_dict["l1"] = l1_loss
+            if self.cfg.use_vae:
+                # Calculate Dₖₗ(latent_pdf || standard_normal). Note: After computing the KL-divergence for
+                # each dimension independently, we sum over the latent dimension to get the total
+                # KL-divergence per batch element, then take the mean over the batch.
+                # (See App. B of https://arxiv.org/abs/1312.6114 for more details).
+                mean_kld = (
+                    (-0.5 * (1 + log_sigma_x2_hat - mu_hat.pow(2) - (log_sigma_x2_hat).exp())).sum(-1).mean()
+                )
+                loss_dict["kl"] = mean_kld
+                loss_dict["loss"] = loss_dict["l1"] + loss_dict["kl"] * self.cfg.kl_weight
+            else:
+                loss_dict["loss"] = loss_dict["l1"]
+            return loss_dict
+        else:
+            action, _ = self._forward(batch["observation.state"], images)
+            return action
+
+    def _forward(
+        self, robot_state: Tensor, image: Tensor, actions: Tensor | None = None
+    ) -> tuple[Tensor, tuple[Tensor | None, Tensor | None]]:
+        """
+        Args:
+            robot_state: (B, J) batch of robot joint configurations.
+            image: (B, N, C, H, W) batch of N camera frames.
+            actions: (B, S, A) batch of actions from the target dataset which must be provided if the
+                VAE is enabled and the model is in training mode.
+        Returns:
+            (B, S, A) batch of action sequences
+            Tuple containing the latent PDF's parameters (mean, log(σ²)) both as (B, L) tensors where L is the
+            latent dimension.
+        """
+        if self.cfg.use_vae and self.training:
+            assert (
+                actions is not None
+            ), "actions must be provided when using the variational objective in training mode."
+
+        batch_size = robot_state.shape[0]
+
+        # Prepare the latent for input to the transformer encoder.
+        if self.cfg.use_vae and actions is not None:
+            # Prepare the input to the VAE encoder: [cls, *joint_space_configuration, *action_sequence].
+            cls_embed = einops.repeat(
+                self.vae_encoder_cls_embed.weight, "1 d -> b 1 d", b=batch_size
+            )  # (B, 1, D)
+            robot_state_embed = self.vae_encoder_robot_state_input_proj(robot_state).unsqueeze(1)  # (B, 1, D)
+            action_embed = self.vae_encoder_action_input_proj(actions)  # (B, S, D)
+            vae_encoder_input = torch.cat([cls_embed, robot_state_embed, action_embed], axis=1)  # (B, S+2, D)
+
+            # Prepare fixed positional embedding.
+            # Note: detach() shouldn't be necessary but leaving it the same as the original code just in case.
+            pos_embed = self.vae_encoder_pos_enc.clone().detach()  # (1, S+2, D)
+
+            # Forward pass through VAE encoder to get the latent PDF parameters.
+            cls_token_out = self.vae_encoder(
+                vae_encoder_input.permute(1, 0, 2), pos_embed=pos_embed.permute(1, 0, 2)
+            )[0]  # select the class token, with shape (B, D)
+            latent_pdf_params = self.vae_encoder_latent_output_proj(cls_token_out)
+            mu = latent_pdf_params[:, : self.latent_dim]
+            # This is 2log(sigma). Done this way to match the original implementation.
+            log_sigma_x2 = latent_pdf_params[:, self.latent_dim :]
+
+            # Sample the latent with the reparameterization trick.
+            latent_sample = mu + log_sigma_x2.div(2).exp() * torch.randn_like(mu)
+        else:
+            # When not using the VAE encoder, we set the latent to be all zeros.
+            mu = log_sigma_x2 = None
+            latent_sample = torch.zeros([batch_size, self.latent_dim], dtype=torch.float32).to(
+                robot_state.device
+            )
+
+        # Prepare all other transformer encoder inputs.
+        # Camera observation features and positional embeddings.
+        all_cam_features = []
+        all_cam_pos_embeds = []
+        for cam_id, _ in enumerate(self.cfg.camera_names):
+            cam_features = self.backbone(image[:, cam_id])["feature_map"]
+            cam_pos_embed = self.encoder_cam_feat_pos_embed(cam_features).to(dtype=cam_features.dtype)
+            cam_features = self.encoder_img_feat_input_proj(cam_features)  # (B, C, h, w)
+            all_cam_features.append(cam_features)
+            all_cam_pos_embeds.append(cam_pos_embed)
+        # Concatenate camera observation feature maps and positional embeddings along the width dimension.
+        encoder_in = torch.cat(all_cam_features, axis=3)
+        cam_pos_embed = torch.cat(all_cam_pos_embeds, axis=3)
+
+        # Get positional embeddings for robot state and latent.
+        robot_state_embed = self.encoder_robot_state_input_proj(robot_state)
+        latent_embed = self.encoder_latent_input_proj(latent_sample)
+
+        # Stack encoder input and positional embeddings moving to (S, B, C).
+        encoder_in = torch.cat(
+            [
+                torch.stack([latent_embed, robot_state_embed], axis=0),
+                encoder_in.flatten(2).permute(2, 0, 1),
+            ]
+        )
+        pos_embed = torch.cat(
+            [
+                self.encoder_robot_and_latent_pos_embed.weight.unsqueeze(1),
+                cam_pos_embed.flatten(2).permute(2, 0, 1),
+            ],
+            axis=0,
+        )
+
+        # Forward pass through the transformer modules.
+        encoder_out = self.encoder(encoder_in, pos_embed=pos_embed)
+        decoder_in = torch.zeros(
+            (self.cfg.chunk_size, batch_size, self.cfg.d_model),
+            dtype=pos_embed.dtype,
+            device=pos_embed.device,
+        )
+        decoder_out = self.decoder(
+            decoder_in,
+            encoder_out,
+            encoder_pos_embed=pos_embed,
+            decoder_pos_embed=self.decoder_pos_embed.weight.unsqueeze(1),
+        )
+
+        # Move back to (B, S, C).
+        decoder_out = decoder_out.transpose(0, 1)
+
+        actions = self.action_head(decoder_out)
+
+        return actions, (mu, log_sigma_x2)
+
+    def save(self, fp):
+        torch.save(self.state_dict(), fp)
+
+    def load(self, fp):
+        d = torch.load(fp)
+        self.load_state_dict(d)
+
+
+class _TransformerEncoder(nn.Module):
+    """Convenience module for running multiple encoder layers, maybe followed by normalization."""
+
+    def __init__(self, cfg: ActConfig):
+        super().__init__()
+        self.layers = nn.ModuleList([_TransformerEncoderLayer(cfg) for _ in range(cfg.n_encoder_layers)])
+        self.norm = nn.LayerNorm(cfg.d_model) if cfg.pre_norm else nn.Identity()
+
+    def forward(self, x: Tensor, pos_embed: Tensor | None = None) -> Tensor:
+        for layer in self.layers:
+            x = layer(x, pos_embed=pos_embed)
+        x = self.norm(x)
+        return x
+
+
+class _TransformerEncoderLayer(nn.Module):
+    def __init__(self, cfg: ActConfig):
+        super().__init__()
+        self.self_attn = nn.MultiheadAttention(cfg.d_model, cfg.n_heads, dropout=cfg.dropout)
+
+        # Feed forward layers.
+        self.linear1 = nn.Linear(cfg.d_model, cfg.dim_feedforward)
+        self.dropout = nn.Dropout(cfg.dropout)
+        self.linear2 = nn.Linear(cfg.dim_feedforward, cfg.d_model)
+
+        self.norm1 = nn.LayerNorm(cfg.d_model)
+        self.norm2 = nn.LayerNorm(cfg.d_model)
+        self.dropout1 = nn.Dropout(cfg.dropout)
+        self.dropout2 = nn.Dropout(cfg.dropout)
+
+        self.activation = _get_activation_fn(cfg.feedforward_activation)
+        self.pre_norm = cfg.pre_norm
+
+    def forward(self, x, pos_embed: Tensor | None = None) -> Tensor:
+        skip = x
+        if self.pre_norm:
+            x = self.norm1(x)
+        q = k = x if pos_embed is None else x + pos_embed
+        x = self.self_attn(q, k, value=x)[0]  # select just the output, not the attention weights
+        x = skip + self.dropout1(x)
+        if self.pre_norm:
+            skip = x
+            x = self.norm2(x)
+        else:
+            x = self.norm1(x)
+            skip = x
+        x = self.linear2(self.dropout(self.activation(self.linear1(x))))
+        x = skip + self.dropout2(x)
+        if not self.pre_norm:
+            x = self.norm2(x)
+        return x
+
+
+class _TransformerDecoder(nn.Module):
+    def __init__(self, cfg: ActConfig):
+        """Convenience module for running multiple decoder layers followed by normalization."""
+        super().__init__()
+        self.layers = nn.ModuleList([_TransformerDecoderLayer(cfg) for _ in range(cfg.n_decoder_layers)])
+        self.norm = nn.LayerNorm(cfg.d_model)
+
+    def forward(
+        self,
+        x: Tensor,
+        encoder_out: Tensor,
+        decoder_pos_embed: Tensor | None = None,
+        encoder_pos_embed: Tensor | None = None,
+    ) -> Tensor:
+        for layer in self.layers:
+            x = layer(
+                x, encoder_out, decoder_pos_embed=decoder_pos_embed, encoder_pos_embed=encoder_pos_embed
+            )
+        if self.norm is not None:
+            x = self.norm(x)
+        return x
+
+
+class _TransformerDecoderLayer(nn.Module):
+    def __init__(self, cfg: ActConfig):
+        super().__init__()
+        self.self_attn = nn.MultiheadAttention(cfg.d_model, cfg.n_heads, dropout=cfg.dropout)
+        self.multihead_attn = nn.MultiheadAttention(cfg.d_model, cfg.n_heads, dropout=cfg.dropout)
+
+        # Feed forward layers.
+        self.linear1 = nn.Linear(cfg.d_model, cfg.dim_feedforward)
+        self.dropout = nn.Dropout(cfg.dropout)
+        self.linear2 = nn.Linear(cfg.dim_feedforward, cfg.d_model)
+
+        self.norm1 = nn.LayerNorm(cfg.d_model)
+        self.norm2 = nn.LayerNorm(cfg.d_model)
+        self.norm3 = nn.LayerNorm(cfg.d_model)
+        self.dropout1 = nn.Dropout(cfg.dropout)
+        self.dropout2 = nn.Dropout(cfg.dropout)
+        self.dropout3 = nn.Dropout(cfg.dropout)
+
+        self.activation = _get_activation_fn(cfg.feedforward_activation)
+        self.pre_norm = cfg.pre_norm
+
+    def maybe_add_pos_embed(self, tensor: Tensor, pos_embed: Tensor | None) -> Tensor:
+        return tensor if pos_embed is None else tensor + pos_embed
+
+    def forward(
+        self,
+        x: Tensor,
+        encoder_out: Tensor,
+        decoder_pos_embed: Tensor | None = None,
+        encoder_pos_embed: Tensor | None = None,
+    ) -> Tensor:
+        """
+        Args:
+            x: (Decoder Sequence, Batch, Channel) tensor of input tokens.
+            encoder_out: (Encoder Sequence, B, C) output features from the last layer of the encoder we are
+                cross-attending with.
+            decoder_pos_embed: (ES, 1, C) positional embedding for keys (from the encoder).
+            encoder_pos_embed: (DS, 1, C) Positional_embedding for the queries (from the decoder).
+        Returns:
+            (DS, B, C) tensor of decoder output features.
+        """
+        skip = x
+        if self.pre_norm:
+            x = self.norm1(x)
+        q = k = self.maybe_add_pos_embed(x, decoder_pos_embed)
+        x = self.self_attn(q, k, value=x)[0]  # select just the output, not the attention weights
+        x = skip + self.dropout1(x)
+        if self.pre_norm:
+            skip = x
+            x = self.norm2(x)
+        else:
+            x = self.norm1(x)
+            skip = x
+        x = self.multihead_attn(
+            query=self.maybe_add_pos_embed(x, decoder_pos_embed),
+            key=self.maybe_add_pos_embed(encoder_out, encoder_pos_embed),
+            value=encoder_out,
+        )[0]  # select just the output, not the attention weights
+        x = skip + self.dropout2(x)
+        if self.pre_norm:
+            skip = x
+            x = self.norm3(x)
+        else:
+            x = self.norm2(x)
+            skip = x
+        x = self.linear2(self.dropout(self.activation(self.linear1(x))))
+        x = skip + self.dropout3(x)
+        if not self.pre_norm:
+            x = self.norm3(x)
+        return x
+
+
+def _create_sinusoidal_position_embedding(num_positions: int, dimension: int) -> Tensor:
+    """1D sinusoidal positional embeddings as in Attention is All You Need.
+
+    Args:
+        num_positions: Number of token positions required.
+    Returns: (num_positions, dimension) position embeddings (the first dimension is the batch dimension).
+
+    """
+
+    def get_position_angle_vec(position):
+        return [position / np.power(10000, 2 * (hid_j // 2) / dimension) for hid_j in range(dimension)]
+
+    sinusoid_table = np.array([get_position_angle_vec(pos_i) for pos_i in range(num_positions)])
+    sinusoid_table[:, 0::2] = np.sin(sinusoid_table[:, 0::2])  # dim 2i
+    sinusoid_table[:, 1::2] = np.cos(sinusoid_table[:, 1::2])  # dim 2i+1
+    return torch.from_numpy(sinusoid_table).float()
+
+
+class _SinusoidalPositionEmbedding2D(nn.Module):
+    """2D sinusoidal positional embeddings similar to what's presented in Attention Is All You Need.
+
+    The variation is that the position indices are normalized in [0, 2π] (not quite: the lower bound is 1/H
+    for the vertical direction, and 1/W for the horizontal direction.
+    """
+
+    def __init__(self, dimension: int):
+        """
+        Args:
+            dimension: The desired dimension of the embeddings.
+        """
+        super().__init__()
+        self.dimension = dimension
+        self._two_pi = 2 * math.pi
+        self._eps = 1e-6
+        # Inverse "common ratio" for the geometric progression in sinusoid frequencies.
+        self._temperature = 10000
+
+    def forward(self, x: Tensor) -> Tensor:
+        """
+        Args:
+            x: A (B, C, H, W) batch of 2D feature map to generate the embeddings for.
+        Returns:
+            A (1, C, H, W) batch of corresponding sinusoidal positional embeddings.
+        """
+        not_mask = torch.ones_like(x[0, :1])  # (1, H, W)
+        # Note: These are like range(1, H+1) and range(1, W+1) respectively, but in most implementations
+        # they would be range(0, H) and range(0, W). Keeping it at as is to match the original code.
+        y_range = not_mask.cumsum(1, dtype=torch.float32)
+        x_range = not_mask.cumsum(2, dtype=torch.float32)
+
+        # "Normalize" the position index such that it ranges in [0, 2π].
+        # Note: Adding epsilon on the denominator should not be needed as all values of y_embed and x_range
+        # are non-zero by construction. This is an artifact of the original code.
+        y_range = y_range / (y_range[:, -1:, :] + self._eps) * self._two_pi
+        x_range = x_range / (x_range[:, :, -1:] + self._eps) * self._two_pi
+
+        inverse_frequency = self._temperature ** (
+            2 * (torch.arange(self.dimension, dtype=torch.float32, device=x.device) // 2) / self.dimension
+        )
+
+        x_range = x_range.unsqueeze(-1) / inverse_frequency  # (1, H, W, 1)
+        y_range = y_range.unsqueeze(-1) / inverse_frequency  # (1, H, W, 1)
+
+        # Note: this stack then flatten operation results in interleaved sine and cosine terms.
+        # pos_embed_x and pos_embed_y are (1, H, W, C // 2).
+        pos_embed_x = torch.stack((x_range[..., 0::2].sin(), x_range[..., 1::2].cos()), dim=-1).flatten(3)
+        pos_embed_y = torch.stack((y_range[..., 0::2].sin(), y_range[..., 1::2].cos()), dim=-1).flatten(3)
+        pos_embed = torch.cat((pos_embed_y, pos_embed_x), dim=3).permute(0, 3, 1, 2)  # (1, C, H, W)
+
+        return pos_embed
+
+
+def _get_activation_fn(activation: str) -> Callable:
+    """Return an activation function given a string."""
+    if activation == "relu":
+        return F.relu
+    if activation == "gelu":
+        return F.gelu
+    if activation == "glu":
+        return F.glu
+    raise RuntimeError(f"activation should be relu/gelu/glu, not {activation}.")
diff --git a/lerobot/common/policies/factory.py b/lerobot/common/policies/factory.py
index 9077d4d0..d9ba3f07 100644
--- a/lerobot/common/policies/factory.py
+++ b/lerobot/common/policies/factory.py
@@ -1,3 +1,8 @@
+import inspect
+
+from lerobot.common.utils import get_safe_torch_device
+
+
 def make_policy(cfg):
     if cfg.policy.name == "tdmpc":
         from lerobot.common.policies.tdmpc.policy import TDMPCPolicy
@@ -21,10 +26,16 @@ def make_policy(cfg):
             **cfg.policy,
         )
     elif cfg.policy.name == "act":
-        from lerobot.common.policies.act.policy import ActionChunkingTransformerPolicy
+        from lerobot.common.policies.act.configuration_act import ActConfig
+        from lerobot.common.policies.act.modeling_act import ActPolicy
 
-        policy = ActionChunkingTransformerPolicy(cfg.policy, cfg.device)
-        policy.to(cfg.device)
+        expected_kwargs = set(inspect.signature(ActConfig).parameters)
+        assert set(cfg.policy).issuperset(
+            expected_kwargs
+        ), f"Hydra config is missing arguments: {set(cfg.policy).difference(expected_kwargs)}"
+        policy_cfg = ActConfig(**{k: v for k, v in cfg.policy.items() if k in expected_kwargs})
+        policy = ActPolicy(policy_cfg)
+        policy.to(get_safe_torch_device(cfg.device))
     else:
         raise ValueError(cfg.policy.name)
 
diff --git a/lerobot/configs/policy/act.yaml b/lerobot/configs/policy/act.yaml
index e2074b46..8ae3087c 100644
--- a/lerobot/configs/policy/act.yaml
+++ b/lerobot/configs/policy/act.yaml
@@ -8,61 +8,64 @@ eval_freq: 10000
 save_freq: 100000
 log_freq: 250
 
-horizon: 100
 n_obs_steps: 1
 # when temporal_agg=False, n_action_steps=horizon
-n_action_steps: ${horizon}
 
 policy:
   name: act
 
   pretrained_model_path:
 
+  # Environment.
+  # Inherit these from the environment.
+  state_dim: ???
+  action_dim: ???
+
+  # Inputs / output structure.
+  n_obs_steps: ${n_obs_steps}
+  camera_names: [top]  # [top, front_close, left_pillar, right_pillar]
+  chunk_size: 100 # chunk_size
+  n_action_steps: 100
+
+  # Vision preprocessing.
+  image_normalization_mean: [0.485, 0.456, 0.406]
+  image_normalization_std: [0.229, 0.224, 0.225]
+
+  # Architecture.
+  # Vision backbone.
+  vision_backbone: resnet18
+  use_pretrained_backbone: true
+  replace_final_stride_with_dilation: false
+  # Transformer layers.
+  pre_norm: false
+  d_model: 512
+  n_heads: 8
+  dim_feedforward: 3200
+  feedforward_activation: relu
+  n_encoder_layers: 4
+  n_decoder_layers: 1
+  # VAE.
+  use_vae: true
+  latent_dim: 32
+  n_vae_encoder_layers: 4
+
+  # Inference.
+  use_temporal_aggregation: false
+
+  # Training and loss computation.
+  dropout: 0.1
+  kl_weight: 10
+
+  # ---
+  # TODO(alexander-soare): Remove these from the policy config.
+  batch_size: 8
   lr: 1e-5
   lr_backbone: 1e-5
-  pretrained_backbone: true
   weight_decay: 1e-4
   grad_clip_norm: 10
-  backbone: resnet18
-  horizon: ${horizon} # chunk_size
-  kl_weight: 10
-  d_model: 512
-  dim_feedforward: 3200
-  vae_enc_layers: 4
-  enc_layers: 4
-  dec_layers: 1
-  num_heads: 8
-  #camera_names: [top, front_close, left_pillar, right_pillar]
-  camera_names: [top]
-  dilation: false
-  dropout: 0.1
-  pre_norm: false
-  activation: relu
-  latent_dim: 32
-
-  use_vae: true
-
-  batch_size: 8
-
-  per_alpha: 0.6
-  per_beta: 0.4
-
-  balanced_sampling: false
   utd: 1
 
-  n_obs_steps: ${n_obs_steps}
-  n_action_steps: ${n_action_steps}
-
-  temporal_agg: false
-
-  state_dim: 14
-  action_dim: 14
-
-  image_normalization:
-    mean: [0.485, 0.456, 0.406]
-    std: [0.229, 0.224, 0.225]
-
   delta_timestamps:
-    observation.images.top: [0.0]
-    observation.state: [0.0]
-    action: "[i / ${fps} for i in range(${horizon})]"
+    observation.images.top: "[i / ${fps} for i in range(1 - ${n_obs_steps}, 1)]"
+    observation.state: "[i / ${fps} for i in range(1 - ${n_obs_steps}, 1)]"
+    action: "[i / ${fps} for i in range(${policy.chunk_size})]"