diff --git a/configs/full_configs/mlm_baseline.yaml b/configs/full_configs/mlm_baseline.yaml
new file mode 100644
index 00000000..c3db9246
--- /dev/null
+++ b/configs/full_configs/mlm_baseline.yaml
@@ -0,0 +1,75 @@
+model:
+  core_model:
+    core_model_type: generic
+    num_layers: 10
+    ffn:
+      ffn_type: swiglu
+      ffn_dim: 1536
+      normalization: rms_norm
+      bias: false
+    attn:
+      attn_type: generic
+      num_heads: 16
+      normalization: rms_norm
+      group_size: 4
+      bias: false
+      is_causal: false
+  embedder:
+    tokenizer_type: gpt2
+    embedding_model_type: generic
+    dataset_name: simple_en_wiki
+  lm_head:
+    normalization: rms_norm
+    bias: false
+    lm_head_type: generic
+  hidden_dim: 512
+  context_window: 512
+  vocab_size: 50257
+  model_shell_type: standard
+  embedding_weight_tying: true
+  positional_encoding_type: rope
+trainer:
+  dropout_scheduler:
+    dropout_type: constant
+    dropout: 0.1
+  dataset: simple_en_wiki
+  training:
+    trainer_type: base_trainer
+    batch_size: 24
+    gradient_accumulation_steps: 20
+    max_iters: 25000
+    lr_decay_iters: 25000
+    warmup_iters: 5000
+    eval_interval: 5000
+    log_interval: 100
+    eval_iters: 200
+    checkpoint_interval: 1000000000.0
+    run_profiler: false
+
+  optimizer:
+    name: nanoGPTadamW
+    lr: 0.0006
+    min_lr: 6.0e-05
+    weight_decay: 0.1
+    beta1: 0.9
+    beta2: 0.95
+    grad_clip: 1.0
+    decay_lr: true
+    warmup_iters: 5000
+  lr_scheduler:
+    name: cosine
+  dataloader:
+    name: mlm
+  loss_fn:
+    name: cross_entropy
+
+general:
+  logging:
+    wandb_log: true
+    wandb_project: SuperTinyLanguageModels
+  paths:
+    output_dir: outputs
+    data_dir: data
+    checkpoint_dir: checkpoints
+  seed: 489
+  device: cuda
diff --git a/trainers/build_trainers.py b/trainers/build_trainers.py
index ff8ff23c..8092852f 100644
--- a/trainers/build_trainers.py
+++ b/trainers/build_trainers.py
@@ -10,6 +10,7 @@
     BytePoolingDataloader,
     NextTokenMLMDataloader,
     ConversationalDataloader,
+    MLMDataloader
 )
 from trainers.loss_fn import (
     cross_entropy_loss_fn,
@@ -86,6 +87,7 @@ def build_dropout_scheduler(trainer_cfg):
     "byte_pooling": BytePoolingDataloader,
     "next_token_mlm": NextTokenMLMDataloader,
     "conversational": ConversationalDataloader,
+    "mlm": MLMDataloader
 }
 
 
diff --git a/trainers/dataloader.py b/trainers/dataloader.py
index 9e26d638..f6de1c71 100644
--- a/trainers/dataloader.py
+++ b/trainers/dataloader.py
@@ -389,3 +389,59 @@ def get_batch(self, split="train", masking_pct=0.15):
         return X, (y, mask)
     
 
+class MLMDataloader(BaseDataloader):
+    """
+    Similarly to the generic dataloader, but mask out some tokens and
+    return the mask used.
+    """
+    def get_batch(self, split="train", masking_pct=0.15):
+        """
+        Get a train/val batch
+        """
+        data = np.memmap(
+            os.path.join(self.tokenized_data_path, f"{split}.bin"),
+            dtype=np.uint16,
+            mode="r",
+        )
+        
+        ## generate the index_ids for X
+        idxs = torch.randint(len(data) - self.context_window, (self.batch_size,))
+        X = torch.stack(
+            [
+                torch.from_numpy((data[i : i + self.context_window]).astype(np.int64))
+                for i in idxs
+            ]
+        )
+
+        ## create a mask of True and False, where True will be Indicies to mask
+        mask = torch.rand(X.size()) < masking_pct
+        # mask &= X != <pad_token_id>
+        # mask &= X != <mask_token_id>
+        # mask &= X != <unk_token_id>
+        ## typically, there won't be any BOS or EOS tokens in the input_ids.
+
+        ## create clones of the input_ids
+        mlm_data = X.clone()
+        labels = X.clone()
+
+        ## get the indices of the mask
+        mask_idx = mask.nonzero(as_tuple=True)
+
+        ## randomise the mask tokens
+        mask_idx_shuffle = torch.randperm(mask_idx[0].size(0))
+
+        ## get the indices of the mask tokens
+        tomask_idx = mask_idx_shuffle[:int(mask_idx[0].shape[0] * 0.8)]
+        torandom_idx = mask_idx_shuffle[int(mask_idx[0].shape[0] * 0.9):]
+
+        ## mask the tokens
+        mlm_data[mask_idx[0][tomask_idx], mask_idx[1][tomask_idx]] = 0
+        mlm_data[mask_idx[0][torandom_idx], mask_idx[1][torandom_idx]] = torch.randint(1, self.vocab_size, (torandom_idx.shape[0],))
+
+        ## create the labels
+        labels[~mask] = -1  ## -1 is the ignore_index
+
+        X = mlm_data.pin_memory().to(self.device, non_blocking=True)
+        y = labels.pin_memory().to(self.device, non_blocking=True)
+
+        return X, y
\ No newline at end of file
diff --git a/trainers/loss_fn.py b/trainers/loss_fn.py
index a956e47f..4889482b 100644
--- a/trainers/loss_fn.py
+++ b/trainers/loss_fn.py
@@ -75,7 +75,7 @@ def compute_perplexity(logits, y, char_lengths, mask=None):
     # flatten both
     logits = logits.view(-1, logits.size(-1))
     y = y.view(-1)
-    loss = torch.nn.functional.cross_entropy(logits, y, reduction="none")
+    loss = torch.nn.functional.cross_entropy(logits, y, reduction="none", ignore_index=-1)
     # B, S, 1
     # unflatten
     loss = loss.view(B, seq_len)