diff --git a/fms_fsdp/config/training.py b/fms_fsdp/config/training.py
index 1d072958..eadef50f 100644
--- a/fms_fsdp/config/training.py
+++ b/fms_fsdp/config/training.py
@@ -72,3 +72,11 @@ class train_config:
     stage2_prompt_length: int = 64
     stage2_batch_size: int = 96
     stage2_seq_length: int = 256
+
+    # FIM training
+    fim_training: bool = False
+    psm_rate: float = 0.0
+    spm_rate: float = 0.0
+    fim_pre: int = 1
+    fim_mid: int = 2
+    fim_suf: int = 3
diff --git a/fms_fsdp/utils/dataloader_utils.py b/fms_fsdp/utils/dataloader_utils.py
index 4b811d6d..d4bbc984 100644
--- a/fms_fsdp/utils/dataloader_utils.py
+++ b/fms_fsdp/utils/dataloader_utils.py
@@ -5,6 +5,7 @@
     AutoHandler,
     BufferDataset,
     CheckpointDataset,
+    FIMDataset,
     ParquetHandler,
     PreloadBufferDataset,
     PreprocessDataset,
@@ -57,9 +58,9 @@ def __iter__(self):
     return torch.utils.data.DataLoader(data, batch_size=cfg.batch_size)
 
 
-def get_data_loader(cfg, rank, world_size, postprocess=[causal_lm]):
+def get_data_loader(cfg, rank, world_size):
     """
-    Pytorch dataloader for stateful, distributed, and rescalable causal language model (CLM) training.
+    Pytorch dataloader for stateful, distributed, and rescalable language model training.
     Assumes underlying data is sequences of integer values.
     ...
     Args
@@ -70,11 +71,11 @@ def get_data_loader(cfg, rank, world_size, postprocess=[causal_lm]):
         Rank of current distributed worker. Used for handling dataset sharding logic.
     world_size : int
         Number of distributed workers. Used for handling dataset sharding logic.
-    postprocess : List[Callable]
-        Any task-specific postprocessing to apply before handing over data. Steps will apply in
-        the order provided by the user. For CLM training, use postprocess=[causal_lm].
     """
 
+    if cfg.fim_training:
+        assert cfg.bos_token is None, "No BOS in FIM training. Did you mean fim_pre?"
+
     datasets, weights = parse_data_args(cfg.datasets, cfg.weights)
 
     # Base streaming dataset. Returns doc chunks in sequence.
@@ -118,9 +119,10 @@ def get_data_loader(cfg, rank, world_size, postprocess=[causal_lm]):
         verbose=(rank == 0),
     )
     # Wrap above dataset in packing logic to form constant-length lines.
+    # Increment seq len to counteract CLM's one token removal.
     data = BufferDataset(
         data,
-        cfg.seq_length if causal_lm not in postprocess else cfg.seq_length + 1,
+        cfg.seq_length + 1,
         bos_token=cfg.bol_token,
         eos_token=cfg.eol_token,
         pack_hard=True,
@@ -128,10 +130,23 @@ def get_data_loader(cfg, rank, world_size, postprocess=[causal_lm]):
     # Shuffle outputs in length 10k buffer. Consecutive lines appear 10k steps apart on average.
     data = PreloadBufferDataset(data, 10000)
 
-    # Apply desired postprocessing steps in sequence
+    # Apply FIM transformation if needed
+    if cfg.fim_training:
+        data = FIMDataset(
+            data,
+            cfg.eos_token,
+            cfg.psm_rate,
+            cfg.spm_rate,
+            pre_token=cfg.fim_pre,
+            mid_token=cfg.fim_mid,
+            suf_token=cfg.fim_suf,
+        )
+
+    # Transform to tensors
     data = PreprocessDataset(data, torch.IntTensor)
-    for p in postprocess:
-        data = PreprocessDataset(data, p)
+
+    # Apply CLM transformation
+    data = PreprocessDataset(data, causal_lm)
 
     # Enable auto-saving
     data = CheckpointDataset(
diff --git a/fms_fsdp/utils/dataset_utils.py b/fms_fsdp/utils/dataset_utils.py
index aedc5862..7d634161 100644
--- a/fms_fsdp/utils/dataset_utils.py
+++ b/fms_fsdp/utils/dataset_utils.py
@@ -696,6 +696,128 @@ def load_state_dict(self, state_dicts, sharded_input=False):
         return sharded_dicts
 
 
+class FIMDataset(_WrapperDataset):
+    """
+    Wrapper for a StatefulDataset that implements Fill-In-the-Middle training
+    (https://arxiv.org/pdf/2207.14255).
+    Input should be a packed sequence (i.e. call BufferDataset before FIMDataset).
+    Breaks sequence apart into component document spans, and for each document span
+    of sufficient length, transforms with specified probability into:
+    PSM mode: <PRE> (prefix) <SUF> (suffix) <MID> (middle) <EOS>
+    SPM mode: <PRE> <SUF> (suffix) <MID> (prefix) (middle) <EOS>
+    The new delimiter tokens can be omitted by passing in None.
+    Any extra tokens after transformation are dropped from the end of the sequence.
+    ...
+    Args
+    ----
+    dataset : _StatefulDataset
+        Fully instantiated dataset
+    delimiter_token : any
+        Token used to indicate document boundaries
+    psm_rate : float
+        Chance to transform into PSM. Cannot exceed 1.
+    spm_rate : float
+        Chance to transform into SPM. Cannot exceed 1.
+    min_len : int
+        Minimum document length to perform FIM transformation
+    pre_token : any | none
+        Token used to indicate prefix section of the document
+    mid_token : any | none
+        Token used to indicate middle infill section of the document
+    suf_token : any | none
+        Token used to indicate suffix section of the document
+    """
+
+    def __init__(
+        self,
+        dataset: _StatefulDataset,
+        delimiter_token: Any,
+        psm_rate: float = 0.0,
+        spm_rate: float = 0.0,
+        min_len: int = 10,
+        pre_token=None,
+        mid_token=None,
+        suf_token=None,
+    ):
+        super().__init__(dataset)
+        assert (
+            psm_rate + spm_rate > 0
+        ), f"FIM training requires SPM or PSM transformation. Please specify a nonzero psm_rate or spm_rate."
+        assert (
+            psm_rate + spm_rate <= 1
+        ), f"Combined psm_rate {psm_rate} and spm_rate {spm_rate} probabilities cannot exceed 1."
+        self.psm = psm_rate
+        self.spm = spm_rate
+        self.delimiter = delimiter_token
+        self.min_len = min_len
+        self.pref = pre_token
+        self.suff = suf_token
+        self.midd = mid_token
+
+        self.g_state = None
+        self.generator = torch.Generator().manual_seed(self.rank)
+        self.state_params = ["g_state"]
+
+    def __iter__(self):
+        dataset = iter(self.dataset)
+        while True:
+            inp = next(dataset)
+            len_ = len(inp)
+            i_eos = [0] + [i for i, x in enumerate(inp) if x == self.delimiter] + [len_]
+            docs = [
+                inp[i_eos[j] + 1 : i_eos[j + 1]] for j in range(len(i_eos) - 1)
+            ]  # list[list[any]]
+            out = []
+            for i in range(len(docs)):
+                doc = docs[i]
+                if len(docs[i]) >= self.min_len:
+                    # decide psm, spm, or nothing
+                    thresh = torch.rand([1], generator=self.generator).item()
+                    if thresh < self.psm + self.spm:
+                        # Split doc
+                        doc = []
+                        if self.pref:
+                            doc = [self.pref]
+                        splits = torch.randint(
+                            0, len(docs[i]), [2], generator=self.generator
+                        ).tolist()
+                        pre = docs[i][: min(splits)]
+                        mid = docs[i][min(splits) : max(splits)]
+                        suf = docs[i][max(splits) :]
+
+                        if thresh < self.psm:
+                            # PSM transformation
+                            doc += pre
+                            if self.suff:
+                                doc.append(self.suff)
+                            doc += suf
+                            if self.midd:
+                                doc.append(self.midd)
+                            doc += mid
+                        else:
+                            # SPM transformation
+                            if self.suff:
+                                doc.append(self.suff)
+                            doc += suf
+                            if self.midd:
+                                doc.append(self.midd)
+                            doc += pre + mid
+                out += doc + [self.delimiter]
+            yield out[:len_]
+
+    def state_dict(self):
+        # Write generator state manually
+        self.g_state = self.generator.get_state()
+        return super().state_dict()
+
+    def load_state_dict(self, state_dicts, sharded_input=False):
+        sharded_dicts = super().load_state_dict(state_dicts, sharded_input)
+        # Manually set generator state if it exists
+        if self.g_state is not None:
+            self.generator.set_state(self.g_state)
+        return sharded_dicts
+
+
 class BufferDataset(_WrapperDataset):
     """
     Wrapper for a _StatefulDataset that takes in sequences of varying lengths, and packs/pads them
@@ -872,9 +994,8 @@ def __init__(
         self.bos = bos_token
         self.drop = strip_tokens
         self.verbose = verbose
-        self.docset: List[
-            Any
-        ] = []  # map of doc indices to (shardid, min docid, max docid)
+        # Map of doc indices to (shardid, min docid, max docid)
+        self.docset: List[Any] = []
 
         # Position
         self.docset_index = 0
diff --git a/main_training_llama.py b/main_training_llama.py
index 67cccee2..a7e1020f 100644
--- a/main_training_llama.py
+++ b/main_training_llama.py
@@ -122,9 +122,11 @@ def main(**kwargs):
         model,
         optimizer,
         None,
-        path=os.path.join(cfg.ckpt_load_path, "checkpoints/")
-        if not os.path.isfile(cfg.ckpt_load_path)
-        else cfg.ckpt_load_path,
+        path=(
+            os.path.join(cfg.ckpt_load_path, "checkpoints/")
+            if not os.path.isfile(cfg.ckpt_load_path)
+            else cfg.ckpt_load_path
+        ),
         strict=False,
     )
     if not is_resuming:
diff --git a/main_training_mamba.py b/main_training_mamba.py
index 3619ea25..68a3c830 100644
--- a/main_training_mamba.py
+++ b/main_training_mamba.py
@@ -119,9 +119,11 @@ def main(**kwargs):
         model,
         optimizer,
         None,
-        path=os.path.join(cfg.ckpt_load_path, "checkpoints/")
-        if not os.path.isfile(cfg.ckpt_load_path)
-        else cfg.ckpt_load_path,
+        path=(
+            os.path.join(cfg.ckpt_load_path, "checkpoints/")
+            if not os.path.isfile(cfg.ckpt_load_path)
+            else cfg.ckpt_load_path
+        ),
         strict=False,
     )
     if not is_resuming:
diff --git a/tests/test_datasets.py b/tests/test_datasets.py
index 83b2426b..40bef481 100644
--- a/tests/test_datasets.py
+++ b/tests/test_datasets.py
@@ -632,6 +632,10 @@ def test_multi_reload_stress():
     # preload / sample / scale / doc pipeline
     multi_reload_stress_check(lambda: d6(d5(d4())))
 
+    # Add FIM dataset
+    d7 = lambda x: [FIMDataset(d, -1, 0.25, 0.25, 10, -2, -3, -4) for d in x]
+    multi_reload_stress_check(lambda: d7(d6(d5(d4()))))
+
 
 # SCALABLEDATASET TESTS