diff --git a/changelog.d/324.change.rst b/changelog.d/324.change.rst
new file mode 100644
index 000000000..11bfd21aa
--- /dev/null
+++ b/changelog.d/324.change.rst
@@ -0,0 +1 @@
+``attr.s`` now auto-detects user-written methods and does not overwrite them. # is missing the __hash__ == None if it shouldn't be hashable
diff --git a/src/attr/_make.py b/src/attr/_make.py
index a8d9c70c7..1328a4037 100644
--- a/src/attr/_make.py
+++ b/src/attr/_make.py
@@ -555,12 +555,18 @@ def slots_setstate(self, state):
         return cls
 
     def add_repr(self, ns):
+        if "__repr__" in self._cls.__dict__:
+            return self
+
         self._cls_dict["__repr__"] = self._add_method_dunders(
             _make_repr(self._attrs, ns=ns)
         )
         return self
 
     def add_str(self):
+        if "__str__" in self._cls.__dict__:
+            return self
+
         repr = self._cls_dict.get("__repr__")
         if repr is None:
             raise ValueError(
@@ -578,13 +584,18 @@ def make_unhashable(self):
         return self
 
     def add_hash(self):
+        if "__hash__" in self._cls.__dict__:
+            return self
+
         self._cls_dict["__hash__"] = self._add_method_dunders(
             _make_hash(self._attrs)
         )
-
         return self
 
     def add_init(self):
+        if "__init__" in self._cls.__dict__:
+            return self
+
         self._cls_dict["__init__"] = self._add_method_dunders(
             _make_init(
                 self._attrs,
@@ -600,11 +611,13 @@ def add_init(self):
     def add_cmp(self):
         cd = self._cls_dict
 
-        cd["__eq__"], cd["__ne__"], cd["__lt__"], cd["__le__"], cd[
-            "__gt__"
-        ], cd["__ge__"] = (
-            self._add_method_dunders(meth) for meth in _make_cmp(self._attrs)
-        )
+        for meth in _make_cmp(self._attrs):
+            method_name = meth.__name__
+
+            if method_name in self._cls.__dict__:
+                continue
+
+            cd[method_name] = self._add_method_dunders(meth)
 
         return self
 
diff --git a/tests/test_make.py b/tests/test_make.py
index 20d13b6a9..1a9c646ff 100644
--- a/tests/test_make.py
+++ b/tests/test_make.py
@@ -422,21 +422,21 @@ def test_adds_all_by_default(self, method_name):
         If no further arguments are supplied, all add_XXX functions except
         add_hash are applied.  __hash__ is set to None.
         """
-        # Set the method name to a sentinel and check whether it has been
-        # overwritten afterwards.
-        sentinel = object()
 
         class C(object):
             x = attr.ib()
 
-        setattr(C, method_name, sentinel)
+        # Assert that the method does not exist yet.
+        assert method_name not in C.__dict__
 
         C = attr.s(C)
-        meth = getattr(C, method_name)
 
-        assert sentinel != meth
+        method = getattr(C, method_name)
+
         if method_name == "__hash__":
-            assert meth is None
+            assert method is None
+        else:
+            assert method is not None
 
     @pytest.mark.parametrize(
         "arg_name, method_name",
@@ -1270,6 +1270,65 @@ class C2(C):
 
         assert [C2] == C.__subclasses__()
 
+    @pytest.mark.parametrize(
+        "method_name",
+        [
+            "__init__",
+            "__hash__",
+            "__repr__",
+            "__str__",
+            "__eq__",
+            "__ne__",
+            "__lt__",
+            "__le__",
+            "__gt__",
+            "__ge__",
+        ],
+    )
+    def test_respect_user_defined_methods(self, method_name):
+        """
+          Does not replace methods provided by the original class.
+        """
+
+        # Set the method name to a sentinel and check that it has not been
+        # overwritten afterwards.
+        def sentinel():
+            pass
+
+        # add_cmp relies on __name__.
+        sentinel.__name__ = method_name
+
+        class C(object):
+            x = attr.ib()
+
+        setattr(C, method_name, sentinel)
+
+        # set sentinel to unbound method of C otherwise assertion for py27
+        # doesn't work
+        sentinel = getattr(C, method_name)
+
+        C = attr.s(C, hash=True, str=(method_name == "__str__"))
+
+        assert sentinel == getattr(C, method_name)
+
+    def test_ignores_user_defined_hash_method(self):
+        """
+          if it should be unhashable, the '__hash__' method is replaced.
+        """
+
+        # Set the method name to a sentinel and check that it has not been
+        # overwritten afterwards.
+        sentinel = object()
+
+        class C(object):
+            x = attr.ib()
+
+        setattr(C, "__hash__", sentinel)
+
+        C = attr.s(C)
+
+        assert getattr(C, "__hash__") is None
+
 
 class TestMakeCmp:
     """