diff --git a/doc/release/1.7.0-notes.rst b/doc/release/1.7.0-notes.rst
index f8f54219cac3..c38f6eff1ec8 100644
--- a/doc/release/1.7.0-notes.rst
+++ b/doc/release/1.7.0-notes.rst
@@ -33,10 +33,14 @@ np.diagonal, numpy 1.7 produces a FutureWarning if it detects
 that you may be attemping to write to such an array. See the documentation
 for array indexing for details.
 
-The default casting rule for UFunc out= parameters has been changed from
-'unsafe' to 'same_kind'.  Most usages which violate the 'same_kind'
-rule are likely bugs, so this change may expose previously undetected
-errors in projects that depend on NumPy.
+In a future version of numpy, the default casting rule for UFunc out=
+parameters will be changed from 'unsafe' to 'same_kind'. (This also
+applies to in-place operations like a += b, which is equivalent to
+np.add(a, b, out=a).) Most usages which violate the 'same_kind' rule
+are likely bugs, so this change may expose previously undetected
+errors in projects that depend on NumPy. In this version of numpy,
+such usages will continue to succeed, but will raise a
+DeprecationWarning.
 
 Full-array boolean indexing has been optimized to use a different,
 optimized code path.   This code path should produce the same results,
diff --git a/doc/source/reference/ufuncs.rst b/doc/source/reference/ufuncs.rst
index 295d52ef4673..afcb1302b32e 100644
--- a/doc/source/reference/ufuncs.rst
+++ b/doc/source/reference/ufuncs.rst
@@ -309,6 +309,12 @@ advanced usage and will not typically be used.
     'equiv', 'safe', 'same_kind', or 'unsafe'. See :func:`can_cast` for
     explanations of the parameter values.
 
+    In a future version of numpy, this argument will default to
+    'same_kind'. As part of this transition, starting in version 1.7,
+    ufuncs will produce a DeprecationWarning for calls which are
+    allowed under the 'unsafe' rules, but not under the 'same_kind'
+    rules.
+
 *order*
 
     .. versionadded:: 1.6
diff --git a/numpy/core/code_generators/numpy_api.py b/numpy/core/code_generators/numpy_api.py
index c49c3c346fbf..cb598880be1b 100644
--- a/numpy/core/code_generators/numpy_api.py
+++ b/numpy/core/code_generators/numpy_api.py
@@ -14,10 +14,12 @@
 
 multiarray_global_vars = {
     'NPY_NUMUSERTYPES':             7,
+    'NPY_DEFAULT_ASSIGN_CASTING':   292,
 }
 
 multiarray_global_vars_types = {
     'NPY_NUMUSERTYPES':             'int',
+    'NPY_DEFAULT_ASSIGN_CASTING':   'NPY_CASTING',
 }
 
 multiarray_scalar_bool_values = {
diff --git a/numpy/core/include/numpy/ndarraytypes.h b/numpy/core/include/numpy/ndarraytypes.h
index 954303352f82..93d561c7c511 100644
--- a/numpy/core/include/numpy/ndarraytypes.h
+++ b/numpy/core/include/numpy/ndarraytypes.h
@@ -199,11 +199,14 @@ typedef enum {
         /* Allow safe casts or casts within the same kind */
         NPY_SAME_KIND_CASTING=3,
         /* Allow any casts */
-        NPY_UNSAFE_CASTING=4
-} NPY_CASTING;
+        NPY_UNSAFE_CASTING=4,
 
-/* The default casting to use for typical assignment operations */
-#define NPY_DEFAULT_ASSIGN_CASTING NPY_SAME_KIND_CASTING
+        /*
+         * Temporary internal definition only, will be removed in upcoming
+         * release, see below
+         * */
+        NPY_INTERNAL_UNSAFE_CASTING_BUT_WARN_UNLESS_SAME_KIND = 100,
+} NPY_CASTING;
 
 typedef enum {
         NPY_CLIP=0,
diff --git a/numpy/core/src/multiarray/common.c b/numpy/core/src/multiarray/common.c
index 5ab8f92bc76b..7b8177c5cb12 100644
--- a/numpy/core/src/multiarray/common.c
+++ b/numpy/core/src/multiarray/common.c
@@ -13,6 +13,22 @@
 #include "common.h"
 #include "buffer.h"
 
+/*
+ * The casting to use for implicit assignment operations resulting from
+ * in-place operations (like +=) and out= arguments. (Notice that this
+ * variable is misnamed, but it's part of the public API so I'm not sure we
+ * can just change it. Maybe someone should try and see if anyone notices.
+ */
+/*
+ * In numpy 1.6 and earlier, this was NPY_UNSAFE_CASTING. In a future
+ * release, it will become NPY_SAME_KIND_CASTING.  Right now, during the
+ * transitional period, we continue to follow the NPY_UNSAFE_CASTING rules (to
+ * avoid breaking people's code), but we also check for whether the cast would
+ * be allowed under the NPY_SAME_KIND_CASTING rules, and if not we issue a
+ * warning (that people's code will be broken in a future release.)
+ */
+NPY_NO_EXPORT NPY_CASTING NPY_DEFAULT_ASSIGN_CASTING = NPY_INTERNAL_UNSAFE_CASTING_BUT_WARN_UNLESS_SAME_KIND;
+
 
 NPY_NO_EXPORT PyArray_Descr *
 _array_find_python_scalar_type(PyObject *op)
diff --git a/numpy/core/src/multiarray/convert_datatype.c b/numpy/core/src/multiarray/convert_datatype.c
index de7468c51e00..586b85b1e9be 100644
--- a/numpy/core/src/multiarray/convert_datatype.c
+++ b/numpy/core/src/multiarray/convert_datatype.c
@@ -503,12 +503,43 @@ type_num_unsigned_to_signed(int type_num)
     }
 }
 
+/*
+ * NOTE: once the UNSAFE_CASTING -> SAME_KIND_CASTING transition is over,
+ * we should remove NPY_INTERNAL_UNSAFE_CASTING_BUT_WARN_UNLESS_SAME_KIND
+ * and PyArray_CanCastTypeTo_impl should be renamed back to
+ * PyArray_CanCastTypeTo.
+ */
+static npy_bool
+PyArray_CanCastTypeTo_impl(PyArray_Descr *from, PyArray_Descr *to,
+                           NPY_CASTING casting);
+
 /*NUMPY_API
  * Returns true if data of type 'from' may be cast to data of type
  * 'to' according to the rule 'casting'.
  */
 NPY_NO_EXPORT npy_bool
 PyArray_CanCastTypeTo(PyArray_Descr *from, PyArray_Descr *to,
+                      NPY_CASTING casting)
+{
+    if (casting == NPY_INTERNAL_UNSAFE_CASTING_BUT_WARN_UNLESS_SAME_KIND) {
+        npy_bool unsafe_ok, same_kind_ok;
+        unsafe_ok = PyArray_CanCastTypeTo_impl(from, to, NPY_UNSAFE_CASTING);
+        same_kind_ok = PyArray_CanCastTypeTo_impl(from, to,
+                                                  NPY_SAME_KIND_CASTING);
+        if (unsafe_ok && !same_kind_ok) {
+            DEPRECATE("Implicitly casting between incompatible kinds. In "
+                      "a future numpy release, this will raise an error. "
+                      "Use casting=\"unsafe\" if this is intentional.");
+        }
+        return unsafe_ok;
+    }
+    else {
+        return PyArray_CanCastTypeTo_impl(from, to, casting);
+    }
+}
+
+static npy_bool
+PyArray_CanCastTypeTo_impl(PyArray_Descr *from, PyArray_Descr *to,
                                                     NPY_CASTING casting)
 {
     /* If unsafe casts are allowed */
diff --git a/numpy/core/tests/test_ufunc.py b/numpy/core/tests/test_ufunc.py
index 53928129f84f..57fd66892938 100644
--- a/numpy/core/tests/test_ufunc.py
+++ b/numpy/core/tests/test_ufunc.py
@@ -742,5 +742,23 @@ def t(expect, func, n, m):
             uf.accumulate(np.zeros((30, 30)), axis=0)
             uf.accumulate(np.zeros((0, 0)), axis=0)
 
+    def test_safe_casting(self):
+        # In old versions of numpy, in-place operations used the 'unsafe'
+        # casting rules. In some future version, 'same_kind' will become the
+        # default.
+        a = np.array([1, 2, 3], dtype=int)
+        # Non-in-place addition is fine
+        assert_array_equal(assert_no_warnings(np.add, a, 1.1),
+                           [2.1, 3.1, 4.1])
+        assert_warns(DeprecationWarning, np.add, a, 1.1, out=a)
+        assert_array_equal(a, [2, 3, 4])
+        def add_inplace(a, b):
+            a += b
+        assert_warns(DeprecationWarning, add_inplace, a, 1.1)
+        assert_array_equal(a, [3, 4, 5])
+        # Make sure that explicitly overriding the warning is allowed:
+        assert_no_warnings(np.add, a, 1.1, out=a, casting="unsafe")
+        assert_array_equal(a, [4, 5, 6])
+
 if __name__ == "__main__":
     run_module_suite()
diff --git a/numpy/testing/tests/test_utils.py b/numpy/testing/tests/test_utils.py
index bff8b50ab5c0..23b2f8e7b228 100644
--- a/numpy/testing/tests/test_utils.py
+++ b/numpy/testing/tests/test_utils.py
@@ -317,11 +317,15 @@ class TestWarns(unittest.TestCase):
     def test_warn(self):
         def f():
             warnings.warn("yo")
+            return 3
 
         before_filters = sys.modules['warnings'].filters[:]
-        assert_warns(UserWarning, f)
+        assert_equal(assert_warns(UserWarning, f), 3)
         after_filters = sys.modules['warnings'].filters
 
+        assert_raises(AssertionError, assert_no_warnings, f)
+        assert_equal(assert_no_warnings(lambda x: x, 1), 1)
+
         # Check that the warnings state is unchanged
         assert_equal(before_filters, after_filters,
                      "assert_warns does not preserver warnings state")
diff --git a/numpy/testing/utils.py b/numpy/testing/utils.py
index ffce2eefc836..16ed0f803a66 100644
--- a/numpy/testing/utils.py
+++ b/numpy/testing/utils.py
@@ -16,7 +16,8 @@
            'decorate_methods', 'jiffies', 'memusage', 'print_assert_equal',
            'raises', 'rand', 'rundocs', 'runstring', 'verbose', 'measure',
            'assert_', 'assert_array_almost_equal_nulp',
-           'assert_array_max_ulp', 'assert_warns', 'assert_allclose']
+           'assert_array_max_ulp', 'assert_warns', 'assert_no_warnings',
+           'assert_allclose']
 
 verbose = 0
 
@@ -1464,7 +1465,7 @@ def assert_warns(warning_class, func, *args, **kw):
 
     Returns
     -------
-    None
+    The value returned by `func`.
 
     """
 
@@ -1474,7 +1475,7 @@ def assert_warns(warning_class, func, *args, **kw):
     l = ctx.__enter__()
     warnings.simplefilter('always')
     try:
-        func(*args, **kw)
+        result = func(*args, **kw)
         if not len(l) > 0:
             raise AssertionError("No warning raised when calling %s"
                     % func.__name__)
@@ -1483,3 +1484,36 @@ def assert_warns(warning_class, func, *args, **kw):
                     "%s( is %s)" % (func.__name__, warning_class, l[0]))
     finally:
         ctx.__exit__()
+    return result
+
+def assert_no_warnings(func, *args, **kw):
+    """
+    Fail if the given callable produces any warnings.
+
+    Parameters
+    ----------
+    func : callable
+        The callable to test.
+    \\*args : Arguments
+        Arguments passed to `func`.
+    \\*\\*kwargs : Kwargs
+        Keyword arguments passed to `func`.
+    
+    Returns
+    -------
+    The value returned by `func`.
+
+    """
+    # XXX: once we may depend on python >= 2.6, this can be replaced by the
+    # warnings module context manager.
+    ctx = WarningManager(record=True)
+    l = ctx.__enter__()
+    warnings.simplefilter('always')
+    try:
+        result = func(*args, **kw)
+        if len(l) > 0:
+            raise AssertionError("Got warnings when calling %s: %s"
+                    % (func.__name__, l))
+    finally:
+        ctx.__exit__()
+    return result