diff --git a/sharding_test.py b/sharding_test.py
new file mode 100644
index 0000000..0c1b3c3
--- /dev/null
+++ b/sharding_test.py
@@ -0,0 +1,88 @@
+import json
+import os
+import shutil
+
+import zarrita
+
+
+shutil.rmtree("sharding_test.zr3", ignore_errors=True)
+h = zarrita.create_hierarchy("sharding_test.zr3")
+a = h.create_array(
+    path="testarray",
+    shape=(20, 3),
+    dtype="float64",
+    chunk_shape=(3, 2),
+    sharding={"chunks_per_shard": (2, 2)},
+)
+
+a[:10, :] = 42
+a[15, 1] = 389
+a[19, 2] = 1
+a[0, 1] = -4.2
+
+assert a.store._chunks_per_shard == (2, 2)
+assert a[15, 1] == 389
+assert a[19, 2] == 1
+assert a[0, 1] == -4.2
+assert a[0, 0] == 42
+
+array_json = a.store["meta/root/testarray.array.json"].decode()
+
+print(array_json)
+# {
+#     "shape": [
+#         20,
+#         3
+#     ],
+#     "data_type": "<f8",
+#     "chunk_grid": {
+#         "type": "regular",
+#         "chunk_shape": [
+#             3,
+#             2
+#         ],
+#         "separator": "/"
+#     },
+#     "chunk_memory_layout": "C",
+#     "fill_value": null,
+#     "extensions": [],
+#     "attributes": {},
+#     "sharding": {
+#         "chunks_per_shard": [
+#             2,
+#             2
+#         ],
+#         "format": "indexed"
+#     }
+# }
+
+assert json.loads(array_json)["sharding"]["chunks_per_shard"] == [2, 2]
+
+print("ONDISK")
+for root, dirs, files in os.walk("sharding_test.zr3"):
+    dirs.sort()
+    if len(files) > 0:
+        print("   ", root.ljust(40), *sorted(files))
+print("UNDERLYING STORE", sorted(i.rsplit("c")[-1] for i in a.store._store if i.startswith("data")))
+print("STORE", sorted(i.rsplit("c")[-1] for i in a.store if i.startswith("data")))
+# ONDISK
+#     sharding_test.zr3                        zarr.json
+#     sharding_test.zr3/data/root/testarray/c0 0
+#     sharding_test.zr3/data/root/testarray/c1 0
+#     sharding_test.zr3/data/root/testarray/c2 0
+#     sharding_test.zr3/data/root/testarray/c3 0
+#     sharding_test.zr3/meta/root              testarray.array.json
+# UNDERLYING STORE ['0/0', '1/0', '2/0', '3/0']
+# STORE ['0/0', '0/1', '1/0', '1/1', '2/0', '2/1', '3/0', '3/1', '5/0', '6/1']
+
+index_bytes = a.store._store["data/root/testarray/c0/0"][-2*2*16:]
+print("INDEX 0.0", [int.from_bytes(index_bytes[i:i+8], byteorder="little") for i in range(0, len(index_bytes), 8)])
+# INDEX 0.0 [0, 48, 48, 48, 96, 48, 144, 48]
+
+
+a_reopened = zarrita.get_hierarchy("sharding_test.zr3").get_array("testarray")
+assert a_reopened.store._chunks_per_shard == (2, 2)
+assert a_reopened[15, 1] == 389
+assert a_reopened[19, 2] == 1
+assert a_reopened[0, 1] == -4.2
+assert a_reopened[0, 0] == 42
diff --git a/zarrita.py b/zarrita.py
index a10214b..37ed72b 100644
--- a/zarrita.py
+++ b/zarrita.py
@@ -3,10 +3,11 @@
 import json
 import numbers
 import itertools
+import functools
 import math
 import re
 from collections.abc import Mapping, MutableMapping
-from typing import Iterator, Union, Optional, Tuple, Any, List, Dict, NamedTuple
+from typing import Iterator, Union, Optional, Tuple, Any, List, Dict, NamedTuple, Iterable, Type
 
 # third-party dependencies
 
@@ -170,6 +171,18 @@ def _check_compressor(compressor: Optional[Codec]) -> None:
     assert compressor is None or isinstance(compressor, Codec)
 
 
+def _check_sharding(sharding: Optional[Dict[str, Any]]) -> Optional[Dict[str, Any]]:
+    if sharding is None:
+        return None
+    if "format" not in sharding:
+        sharding["format"] = "indexed"
+    assert sharding["format"] in SHARDED_STORES, (
+        f"Shard format {sharding['format']} is not supported, "
+        + f"use one of {list(SHARDED_STORES)}"
+    )
+    return sharding
+
+
 def _encode_codec_metadata(codec: Codec) -> Optional[Mapping]:
     if codec is None:
         return None
@@ -265,7 +278,8 @@ def create_array(self,
                      chunk_separator: str = "/",
                      compressor: Optional[Codec] = None,
                      fill_value: Any = None,
-                     attrs: Optional[Mapping] = None) -> Array:
+                     attrs: Optional[Mapping] = None,
+                     sharding: Optional[Dict[str, Any]] = None) -> Array:
 
         # sanity checks
         path = _check_path(path)
@@ -274,6 +288,7 @@ def create_array(self,
         chunk_shape = _check_chunk_shape(chunk_shape, shape)
         _check_compressor(compressor)
         attrs = _check_attrs(attrs)
+        sharding = _check_sharding(sharding)
 
         # encode data type
         if dtype == np.bool_:
@@ -297,6 +312,8 @@ def create_array(self,
         )
         if compressor is not None:
             meta["compressor"] = _encode_codec_metadata(compressor)
+        if sharding is not None:
+            meta["sharding"] = sharding
 
         # serialise and store metadata document
         meta_doc = _json_encode_object(meta)
@@ -307,7 +324,8 @@ def create_array(self,
         array = Array(store=self.store, path=path, owner=self,
                       shape=shape, dtype=dtype, chunk_shape=chunk_shape,
                       chunk_separator=chunk_separator, compressor=compressor,
-                      fill_value=fill_value, attrs=attrs)
+                      fill_value=fill_value, attrs=attrs,
+                      sharding=sharding)
 
         return array
 
@@ -341,12 +359,13 @@ def get_array(self, path: str) -> Array:
             if spec["must_understand"]:
                 raise NotImplementedError(spec)
         attrs = meta["attributes"]
+        sharding = meta.get("sharding", None)
 
         # instantiate array
         a = Array(store=self.store, path=path, owner=self, shape=shape,
                   dtype=dtype, chunk_shape=chunk_shape,
                   chunk_separator=chunk_separator, compressor=compressor,
-                  fill_value=fill_value, attrs=attrs)
+                  fill_value=fill_value, attrs=attrs, sharding=sharding)
 
         return a
 
@@ -587,7 +606,15 @@ def __init__(self,
                  chunk_separator: str,
                  compressor: Optional[Codec],
                  fill_value: Any = None,
-                 attrs: Optional[Mapping] = None):
+                 attrs: Optional[Mapping] = None,
+                 sharding: Optional[Dict[str, Any]] = None,
+                 ):
+        if sharding is not None:
+            store = SHARDED_STORES[sharding["format"]](  # type: ignore
+                store=store,
+                chunk_separator=chunk_separator,
+                **sharding,
+            )
         super().__init__(store=store, path=path, owner=owner)
         self.shape = shape
         self.dtype = dtype
@@ -771,7 +798,7 @@ def _chunk_setitem(self, chunk_coords, chunk_selection, value):
         encoded_chunk_data = self._encode_chunk(chunk)
 
         # store
-        self.store[chunk_key] = encoded_chunk_data
+        self.store[chunk_key] = encoded_chunk_data.tobytes()
 
     def _encode_chunk(self, chunk):
 
@@ -1146,3 +1173,176 @@ def __repr__(self) -> str:
         if isinstance(protocol, tuple):
             protocol = protocol[-1]
         return f"{protocol}://{self.root}"
+
+
+MAX_UINT_64 = 2 ** 64 - 1
+
+
+def _is_data_key(key: str) -> bool:
+    return key.startswith("data/root")
+
+
+class _ShardIndex(NamedTuple):
+    store: "IndexedShardedStore"
+    offsets_and_lengths: np.ndarray  # dtype uint64, shape (chunks_per_shard_0, chunks_per_shard_1, ..., 2)
+
+    def __localize_chunk__(self, chunk: Tuple[int, ...]) -> Tuple[int, ...]:
+        return tuple(chunk_i % shard_i for chunk_i, shard_i in zip(chunk, self.store._chunks_per_shard))
+
+    def get_chunk_slice(self, chunk: Tuple[int, ...]) -> Optional[slice]:
+        localized_chunk = self.__localize_chunk__(chunk)
+        chunk_start, chunk_len = self.offsets_and_lengths[localized_chunk]
+        if (chunk_start, chunk_len) == (MAX_UINT_64, MAX_UINT_64):
+            return None
+        else:
+            return slice(chunk_start, chunk_start + chunk_len)
+
+    def set_chunk_slice(self, chunk: Tuple[int, ...], chunk_slice: Optional[slice]) -> None:
+        localized_chunk = self.__localize_chunk__(chunk)
+        if chunk_slice is None:
+            self.offsets_and_lengths[localized_chunk] = (MAX_UINT_64, MAX_UINT_64)
+        else:
+            self.offsets_and_lengths[localized_chunk] = (
+                chunk_slice.start,
+                chunk_slice.stop - chunk_slice.start
+            )
+
+    def to_bytes(self) -> bytes:
+        return self.offsets_and_lengths.tobytes(order='C')
+
+    @classmethod
+    def from_bytes(
+        cls, buffer: Union[bytes, bytearray], store: "IndexedShardedStore"
+    ) -> "_ShardIndex":
+        return cls(
+            store=store,
+            offsets_and_lengths=np.frombuffer(
+                bytearray(buffer), dtype="<u8"
+            ).reshape(*store._chunks_per_shard, 2, order="C")
+        )
+
+    @classmethod
+    def create_empty(cls, store: "IndexedShardedStore"):
+        # reserving 2*64bit per chunk for offset and length:
+        return cls.from_bytes(
+            MAX_UINT_64.to_bytes(8, byteorder="little") * (2 * store._num_chunks_per_shard),
+            store=store
+        )
+
+
+class IndexedShardedStore(Store):
+    """This class should not be used directly,
+    but is added to an Array as a wrapper when needed automatically."""
+
+    def __init__(
+        self,
+        store: Store,
+        chunk_separator: str,
+        chunks_per_shard: Iterable[int],
+        **kwargs: Any,
+    ) -> None:
+        self._store = store
+        self._num_chunks_per_shard = functools.reduce(lambda x, y: x*y, chunks_per_shard, 1)
+        self._chunk_separator = chunk_separator
+        assert all(isinstance(s, int) for s in chunks_per_shard)
+        self._chunks_per_shard = tuple(chunks_per_shard)
+
+    def _key_to_shard(
+        self, chunk_key: str
+    ) -> Tuple[str, Tuple[int, ...]]:
+        prefix, _, chunk_string = chunk_key.rpartition("c")
+        chunk_subkeys = tuple(map(int, chunk_string.split(self._chunk_separator)))
+        shard_key_tuple = (
+            subkey // shard_i for subkey, shard_i in zip(chunk_subkeys, self._chunks_per_shard)
+        )
+        shard_key = prefix + "c" + self._chunk_separator.join(map(str, shard_key_tuple))
+        return shard_key, chunk_subkeys
+
+    def _get_index(self, buffer: Union[bytes, bytearray]) -> _ShardIndex:
+        # At the end of each shard 2*64bit per chunk for offset and length define the index:
+        return _ShardIndex.from_bytes(buffer[-16 * self._num_chunks_per_shard:], self)
+
+    def _get_chunks_in_shard(self, shard_key: str) -> Iterator[Tuple[int, ...]]:
+        _, _, chunk_string = shard_key.rpartition("c")
+        shard_key_tuple = tuple(map(int, chunk_string.split(self._chunk_separator)))
+        for chunk_offset in itertools.product(*(range(i) for i in self._chunks_per_shard)):
+            yield tuple(
+                shard_key_i * shards_i + offset_i
+                for shard_key_i, offset_i, shards_i
+                in zip(shard_key_tuple, chunk_offset, self._chunks_per_shard)
+            )
+
+    def __getitem__(self, key: str, default: Optional[bytes] = None) -> bytes:
+        if _is_data_key(key):
+            shard_key, chunk_subkeys = self._key_to_shard(key)
+            full_shard_value = self._store[shard_key]
+            index = self._get_index(full_shard_value)
+            chunk_slice = index.get_chunk_slice(chunk_subkeys)
+            if chunk_slice is not None:
+                return full_shard_value[chunk_slice]
+            else:
+                if default is not None:
+                    return default
+                raise KeyError(key)
+        else:
+            return self._store.__getitem__(key, default)
+
+    def __setitem__(self, key: str, value: bytes) -> None:
+        if _is_data_key(key):
+            shard_key, chunk_subkeys = self._key_to_shard(key)
+            chunks_to_read = set(self._get_chunks_in_shard(shard_key))
+            chunks_to_read.remove(chunk_subkeys)
+            new_content = {chunk_subkeys: value}
+            try:
+                full_shard_value = self._store[shard_key]
+            except KeyError:
+                index = _ShardIndex.create_empty(self)
+            else:
+                index = self._get_index(full_shard_value)
+                for chunk_to_read in chunks_to_read:
+                    chunk_slice = index.get_chunk_slice(chunk_to_read)
+                    if chunk_slice is not None:
+                        new_content[chunk_to_read] = full_shard_value[chunk_slice]
+
+            shard_content = b""
+            for chunk_subkeys, chunk_content in new_content.items():
+                chunk_slice = slice(len(shard_content), len(shard_content) + len(chunk_content))
+                index.set_chunk_slice(chunk_subkeys, chunk_slice)
+                shard_content += chunk_content
+            # Appending the index at the end of the shard:
+            shard_content += index.to_bytes()
+            self._store[shard_key] = shard_content
+        else:
+            self._store[key] = value
+
+    def _shard_key_to_original_keys(self, key: str) -> Iterator[str]:
+        if not _is_data_key(key):
+            # Special keys such as meta-keys are passed on as-is
+            yield key
+        else:
+            index = self._get_index(self._store[key])
+            prefix, _, _ = key.rpartition("c")
+            for chunk_tuple in self._get_chunks_in_shard(key):
+                if index.get_chunk_slice(chunk_tuple) is not None:
+                    yield prefix + "c" + self._chunk_separator.join(map(str, chunk_tuple))
+
+    def __iter__(self) -> Iterator[str]:
+        for key in self._store:
+            yield from self._shard_key_to_original_keys(key)
+
+    def list_prefix(self, prefix: str) -> List[str]:
+        if _is_data_key(prefix):
+            # Needs translation of the prefix to shard_key
+            raise NotImplementedError
+        return self._store.list_prefix(prefix)
+
+    def list_dir(self, prefix: str) -> ListDirResult:
+        if _is_data_key(prefix):
+            # Needs translation of the prefix to shard_key
+            raise NotImplementedError
+        return self._store.list_dir(prefix)
+
+
+SHARDED_STORES: Dict[str, Type[Store]] = {
+    "indexed": IndexedShardedStore,
+}