diff --git a/flax/cursor.py b/flax/cursor.py
new file mode 100644
index 0000000000..d38e24a0c6
--- /dev/null
+++ b/flax/cursor.py
@@ -0,0 +1,213 @@
+import enum
+from sqlite3 import Cursor
+from typing import Any, Callable, Generic, Mapping, NamedTuple, Optional, Protocol, Sequence, Tuple, TypeVar, Union, runtime_checkable
+import jax
+from jax._src.tree_util import SequenceKey, DictKey, GetAttrKey, FlattenedIndexKey
+from flax.core import freeze, FrozenDict
+import dataclasses
+from flax.training.train_state import TrainState
+import optax
+
+A = TypeVar('A')
+Key = Any
+
+
+@runtime_checkable
+class Indexable(Protocol):
+
+  def __getitem__(self, key) -> Any:
+    ...
+
+
+class AccessType(enum.Enum):
+  GETATTR = enum.auto()
+  GETITEM = enum.auto()
+
+
+# def flatten_until_found(tree: Any, targets: Sequence[Any]):
+#   remaining = len(targets)
+
+#   def is_leaf(x):
+#     nonlocal remaining
+#     leaf = False
+#     if x in targets:
+#       remaining -= 1
+#       leaf = True
+#     return leaf or remaining <= 0
+
+#   value = jax.tree_util.tree_flatten(tree, is_leaf=is_leaf)
+
+#   if remaining > 0:
+#     return None
+#   else:
+#     return value
+
+
+@dataclasses.dataclass
+class ParentKey(Generic[A]):
+  parent: 'Cursor[A]'
+  key: Any
+
+
+class Cursor(Generic[A]):
+  obj: A
+  parent_key: Optional[ParentKey[A]]
+  changes: dict[Any, Union[Any, 'Cursor[A]']]
+
+  def __init__(self, obj: A, parent_key: Optional[ParentKey[A]]):
+    # NOTE: we use `vars` here to avoid calling `__setattr__`
+    # vars(self) = self.__dict__
+    vars(self)['obj'] = obj
+    vars(self)['parent_key'] = parent_key
+    vars(self)['changes'] = {}
+
+  @property
+  def root(self) -> 'Cursor[A]':
+    if self.parent_key is None:
+      return self
+    else:
+      return self.parent_key.parent.root
+
+  def __getitem__(self, key) -> 'Cursor[A]':
+    if key in self.changes:
+      return self.changes[key]
+
+    if not isinstance(self.obj, Indexable):
+      raise TypeError(f'Cannot index into {self.obj}')
+
+    if isinstance(self.obj, Mapping) and key not in self.obj:
+      raise KeyError(f'Key {key} not found in {self.obj}')
+
+    child = Cursor(self.obj[key], ParentKey(self, key))
+    self.changes[key] = child
+    return child
+
+  def __getattr__(self, name) -> 'Cursor[A]':
+    if name in self.changes:
+      return self.changes[name]
+
+    if not hasattr(self.obj, name):
+      raise AttributeError(f'Attribute {name} not found in {self.obj}')
+
+    child = Cursor(getattr(self.obj, name), ParentKey(self, name))
+    self.changes[name] = child
+    return child
+
+  def __setitem__(self, key, value):
+    self.changes[key] = value
+
+  def __setattr__(self, name, value):
+    self.changes[name] = value
+
+  def apply_filter(
+      self, filter_fn: Callable[[Sequence[str], Any], Tuple[bool, Any]]
+  ):
+    """
+    def increment_ints_at_layer1(path: str, value):
+      if 'layer1' in path and isinstance(value, int)
+        return True, value + 1
+      else:
+        return False, value
+
+    c = cursor(config)
+    c.apply_filter(increment_ints_at_layer1)
+    config = c.build()
+    """
+
+    def get_child(obj, key):
+      if isinstance(key, SequenceKey):
+        return obj[key.idx]
+      if isinstance(key, (DictKey, FlattenedIndexKey)):
+        return obj[key.key]
+      if isinstance(key, GetAttrKey):
+        return getattr(obj, key.name)
+      raise KeyError(
+          f'Key {key} of type {type(key)} is not a valid key type. Must be one'
+          ' of [SequenceKey, GetAttrKey, DictKey, FlattenedIndexKey].'
+      )
+
+    def set_child(obj, key, value):
+      if isinstance(key, SequenceKey):
+        obj[key.idx] = value
+      elif isinstance(key, (DictKey, FlattenedIndexKey)):
+        obj[key.key] = value
+      elif isinstance(key, GetAttrKey):
+        setattr(obj, key.name, value)
+      else:
+        raise KeyError(
+            f'Key {key} of type {type(key)} is not a valid key type. Must be'
+            ' one of [SequenceKey, GetAttrKey, DictKey, FlattenedIndexKey].'
+        )
+
+    def get_key_str(key):
+      if isinstance(key, SequenceKey):
+        return str(key.idx)
+      elif isinstance(key, (DictKey, FlattenedIndexKey)):
+        return str(key.key)
+      elif isinstance(key, GetAttrKey):
+        return str(key.name)
+      else:
+        raise KeyError(
+            f'Key {key} of type {type(key)} is not a valid key type. Must be'
+            ' one of [SequenceKey, GetAttrKey, DictKey, FlattenedIndexKey].'
+        )
+
+    flattened, _ = jax.tree_util.tree_flatten_with_path(self.obj)
+    for key_path, value in flattened:
+      valid, new_value = filter_fn(
+          [get_key_str(key) for key in key_path], value
+      )
+      if valid:
+        obj = self
+        for key in key_path[:-1]:
+          obj = get_child(obj, key)
+        set_child(obj, key_path[-1], new_value)
+
+  def build(self) -> A:
+    changes = {
+        key: child.build() if isinstance(child, Cursor) else child
+        for key, child in self.changes.items()
+    }
+    if isinstance(self.obj, FrozenDict):
+      obj = self.obj.copy(changes)
+    elif isinstance(self.obj, (dict, list)):
+      obj = self.obj.copy()
+      for key, value in changes.items():
+        obj[key] = value
+    elif isinstance(self.obj, tuple):
+      obj = list(self.obj)
+      for key, value in changes.items():
+        obj[key] = value
+      obj = tuple(obj)
+    elif dataclasses.is_dataclass(self.obj):
+      obj = dataclasses.replace(self.obj, **changes)
+    else:
+      raise ValueError(f'Cannot build object of type {type(self.obj).__name__}')
+
+      # NOTE: There is a way to try to do a general replace for pytrees, but it requires
+      # the key of `changes` to store the type of access (getattr, getitem, etc.)
+      # in order to access those value from the original object and try to replace them
+      # with the new value. For simplicity, this is not implemented for now.
+      # ----------------------
+      # changed_values = tuple(changes.values())
+      # result = flatten_until_found(self.obj, changed_values)
+
+      # if result is None:
+      #   raise ValueError('Cannot find object in parent')
+
+      # leaves, treedef = result
+      # leaves = [leaf if leaf is not self.obj else value for leaf in leaves]
+      # obj = jax.tree_util.tree_unflatten(treedef, leaves)
+
+    return obj  # type: ignore
+
+  def set(self, value) -> A:
+    if self.parent_key is None:
+      return value
+    parent, key = self.parent_key.parent, self.parent_key.key
+    parent.changes[key] = value
+    return parent.root.build()
+
+
+def cursor(obj: A) -> Cursor[A]:
+  return Cursor(obj, None)
diff --git a/tests/cursor_test.py b/tests/cursor_test.py
new file mode 100644
index 0000000000..56851e7a95
--- /dev/null
+++ b/tests/cursor_test.py
@@ -0,0 +1,137 @@
+# Copyright 2023 The Flax Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests for flax.struct."""
+
+
+from absl.testing import absltest
+import jax
+import jax.numpy as jnp
+import optax
+
+import flax.linen as nn
+from flax.core import freeze
+from flax.cursor import cursor
+from flax.training import train_state
+
+# Parse absl flags test_srcdir and test_tmpdir.
+jax.config.parse_flags_with_absl()
+
+
+class CursorTest(absltest.TestCase):
+
+  def test_set_and_build(self):
+    # test regular dicts and FrozenDicts
+    dict_obj = {'a': 1, 'b': (2, 3), 'c': [4, 5]}
+    for d, freeze_wrap in ((dict_obj, lambda x: x), (freeze(dict_obj), freeze)):
+      # set API
+      self.assertEqual(
+          cursor(d)['b'][0].set(10),
+          freeze_wrap({'a': 1, 'b': (10, 3), 'c': [4, 5]}),
+      )
+      # build API
+      c = cursor(d)
+      c['b'][0] = 20
+      c['a'] = (100, 200)
+      d2 = c.build()
+      self.assertEqual(
+          d2, freeze_wrap({'a': (100, 200), 'b': (20, 3), 'c': [4, 5]})
+      )
+
+    # test lists and tuples
+    list_obj = [0, dict_obj, (1, 2), [3, 4, 5]]
+    for l, tuple_wrap in ((list_obj, lambda x: x), (tuple(list_obj), tuple)):
+      # set API
+      self.assertEqual(
+          cursor(l)[1]['b'][0].set(10),
+          tuple_wrap(
+              [0, {'a': 1, 'b': (10, 3), 'c': [4, 5]}, (1, 2), [3, 4, 5]]
+          ),
+      )
+      # build API
+      c = cursor(l)
+      c[1]['b'][0] = 20
+      c[2] = (100, 200)
+      l2 = c.build()
+      self.assertEqual(
+          l2,
+          tuple_wrap(
+              [0, {'a': 1, 'b': (20, 3), 'c': [4, 5]}, (100, 200), [3, 4, 5]]
+          ),
+      )
+
+    # test TrainState
+    state = train_state.TrainState.create(
+        apply_fn=lambda x: x,
+        params=dict_obj,
+        tx=optax.adam(1e-3),
+    )
+    # set API
+    self.assertEqual(
+        cursor(state).params['b'][0].set(10).params,
+        {'a': 1, 'b': (10, 3), 'c': [4, 5]},
+    )
+    # build API
+    new_fn = lambda x: x + 1
+    c = cursor(state)
+    c.apply_fn = new_fn
+    c.params['b'][0] = 20
+    c.params['a'] = (100, 200)
+    state2 = c.build()
+    self.assertEqual(state2.apply_fn, new_fn)
+    self.assertEqual(
+        state2.params, {'a': (100, 200), 'b': (20, 3), 'c': [4, 5]}
+    )
+
+  def test_apply_filter(self):
+    class Model(nn.Module):
+
+      @nn.compact
+      def __call__(self, x):
+        x = nn.Dense(3)(x)
+        x = nn.relu(x)
+        x = nn.Dense(3)(x)
+        x = nn.relu(x)
+        x = nn.Dense(3)(x)
+        x = nn.relu(x)
+        return x
+
+    params = Model().init(jax.random.PRNGKey(0), jnp.empty((1, 2)))['params']
+
+    def filter_fn(path, value):
+      if 'kernel' in path:
+        return True, value * 2 + 1
+      elif 'Dense_1' in path and 'bias' in path:
+        return True, value - 1
+      return False, value
+
+    c = cursor(params)
+    c.apply_filter(filter_fn)
+    new_params = c.build()
+    for layer in ('Dense_0', 'Dense_1', 'Dense_2'):
+      self.assertTrue(
+          (new_params[layer]['kernel'] == 2 * params[layer]['kernel'] + 1).all()
+      )
+      if layer == 'Dense_1':
+        self.assertTrue(
+            (new_params[layer]['bias'] == jnp.array([-1, -1, -1])).all()
+        )
+      else:
+        self.assertTrue(
+            (new_params[layer]['bias'] == params[layer]['bias']).all()
+        )
+
+
+if __name__ == '__main__':
+  absltest.main()