diff --git a/CHANGELOG b/CHANGELOG
index 5fb54f1170..bd410367ed 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,4 @@
+* Add Len to PYSON
 * Use bcrypt to hash password if possible
 * Use a sequence of ids, values to set fields
 * Client side actions on button and wizard
diff --git a/doc/ref/pyson.rst b/doc/ref/pyson.rst
index 5f012b91a7..fd1640be50 100644
--- a/doc/ref/pyson.rst
+++ b/doc/ref/pyson.rst
@@ -250,6 +250,11 @@ Arguments:
 ``delta_microseconds``
     Contains a PYSON statement of type int or long.
 
+.. class:: Len(value)
+
+A :class:`Len` object represents the PYSON ``Len()`` statement for length of a
+dictionary, list or string. Returns the number of items in ``value``.
+
 .. class:: Id(module, fs_id)
 
 An :class:`Id` object represents the PYSON ``Id()`` statement for filesystem id
diff --git a/trytond/pyson.py b/trytond/pyson.py
index 1fc1e6440d..6771f20ce6 100644
--- a/trytond/pyson.py
+++ b/trytond/pyson.py
@@ -537,6 +537,32 @@ class DateTime(Date):
             )
 
 
+class Len(PYSON):
+
+    def __init__(self, value):
+        super(Len, self).__init__()
+        if isinstance(value, PYSON):
+            assert value.types().issubset(set([dict, list, str])), \
+                'value must be a dict or a list or a string'
+        else:
+            assert type(value) in [dict, list, str], \
+                'value must be a dict or list or a string'
+        self._value = value
+
+    def pyson(self):
+        return {
+            '__class__': 'Len',
+            'v': self._value,
+            }
+
+    def types(self):
+        return set([int, long])
+
+    @staticmethod
+    def eval(dct, context):
+        return len(dct['v'])
+
+
 class Id(PYSON):
     """The database id for filesystem id"""
 
@@ -567,4 +593,5 @@ CONTEXT = {
     'In': In,
     'Date': Date,
     'DateTime': DateTime,
+    'Len': Len,
 }
diff --git a/trytond/tests/test_pyson.py b/trytond/tests/test_pyson.py
index d49c7ed6e5..d295b98021 100644
--- a/trytond/tests/test_pyson.py
+++ b/trytond/tests/test_pyson.py
@@ -496,6 +496,26 @@ class PYSONTestCase(unittest.TestCase):
         self.assert_(pyson.PYSONDecoder().decode(eval)
                 == datetime.datetime(2010, 2, 22, 10, 30, 20, 200))
 
+    def test0140Len(self):
+        'Test pyson.Len'
+        self.assert_(pyson.Len([1, 2, 3]).pyson() == {
+                '__class__': 'Len',
+                'v': [1, 2, 3],
+                })
+
+        self.assertRaises(AssertionError, pyson.Len, object())
+
+        self.assert_(pyson.Len([1, 2, 3]).types() == set([int, long]))
+
+        eval = pyson.PYSONEncoder().encode(pyson.Len([1, 2, 3]))
+        self.assertEqual(pyson.PYSONDecoder().decode(eval), 3)
+
+        eval = pyson.PYSONEncoder().encode(pyson.Len({1: 2, 3: 4}))
+        self.assertEqual(pyson.PYSONDecoder().decode(eval), 2)
+
+        eval = pyson.PYSONEncoder().encode(pyson.Len('foo bar'))
+        self.assertEqual(pyson.PYSONDecoder().decode(eval), 7)
+
     def test0900Composite(self):
         'Test Composite'
         eval = pyson.PYSONEncoder().encode(['id', pyson.If(pyson.Not(