diff --git a/cacheflow/sequence.py b/cacheflow/sequence.py
new file mode 100644
index 00000000..172b0b6b
--- /dev/null
+++ b/cacheflow/sequence.py
@@ -0,0 +1,78 @@
+import enum
+from typing import List, Optional
+
+from cacheflow.block import LogicalTokenBlock
+from cacheflow.decoding import DecodingParams
+
+
+class SequenceStatus(enum.Enum):
+    PENDING = enum.auto()
+    SERVING = enum.auto()
+    SWAPPED = enum.auto()
+    FINISHED = enum.auto()
+
+
+class Sequence:
+
+    def __init__(
+        self,
+        seq_id: int,
+        token_ids: List[int],
+        max_context_len: int,
+        block_size: int,
+    ) -> None:
+        self.seq_id = seq_id
+        self.max_context_len = max_context_len
+        self.block_size = block_size
+
+        self.logical_token_blocks: List[LogicalTokenBlock] = []
+        # Initialize the logical token blocks with the given token ids.
+        self.append(token_ids)
+
+        self.status = SequenceStatus.PENDING
+
+    def add_block(self) -> None:
+        block = LogicalTokenBlock(
+            block_number=len(self.logical_token_blocks),
+            block_size=self.block_size,
+        )
+        self.logical_token_blocks.append(block)
+
+    def append(self, token_ids: List[int]) -> None:
+        while token_ids:
+            if not self.logical_token_blocks:
+                self.add_block()
+
+            last_block = self.logical_token_blocks[-1]
+            if last_block.is_full():
+                self.add_block()
+                last_block = self.logical_token_blocks[-1]
+
+            num_empty_slots = last_block.get_num_empty_slots()
+            last_block.append(token_ids[:num_empty_slots])
+            token_ids = token_ids[num_empty_slots:]
+
+    def get_token_ids(self) -> List[int]:
+        token_ids: List[int] = []
+        for block in self.logical_token_blocks:
+            token_ids.extend(block.get_token_ids())
+        return token_ids
+
+
+class SequenceGroup:
+
+    def __init__(
+        self,
+        group_id: int,
+        seqs: List[Sequence],
+        decoding_params: DecodingParams,
+    ) -> None:
+        self.group_id = group_id
+        self.seqs = seqs
+        self.decoding_params = decoding_params
+
+    def num_seqs(self, status: Optional[SequenceStatus] = None) -> int:
+        if status is None:
+            return len(self.seqs)
+        else:
+            return len([seq for seq in self.seqs if seq.status == status])