diff --git a/RELEASE.rst b/RELEASE.rst
index 307986ab81681..e4383d3bfcca8 100644
--- a/RELEASE.rst
+++ b/RELEASE.rst
@@ -79,6 +79,8 @@ pandas 0.11.1
     spurious plots from showing up.
   - Added Faq section on repr display options, to help users customize their setup.
   - ``where`` operations that result in block splitting are much faster (GH3733_)
+  - ``groupby`` will now warn with a ``PerformanceWarning`` if an aggregate function
+    returns an array or list, instead of raising an error. (GH3788_)
 
 **API Changes**
 
@@ -312,6 +314,7 @@ pandas 0.11.1
 .. _GH3726: https://github.com/pydata/pandas/issues/3726
 .. _GH3795: https://github.com/pydata/pandas/issues/3795
 .. _GH3814: https://github.com/pydata/pandas/issues/3814
+.. _GH3788: https://github.com/pydata/pandas/issues/3788
 
 pandas 0.11.0
 =============
diff --git a/pandas/core/common.py b/pandas/core/common.py
index 69f38bf0c7c61..b4395cc6e3bf1 100644
--- a/pandas/core/common.py
+++ b/pandas/core/common.py
@@ -42,6 +42,10 @@ class AmbiguousIndexError(PandasError, KeyError):
     pass
 
 
+class PerformanceWarning(Warning):
+    "Baseclass for warnings about performance issues that affect speed, but not functionality."
+    pass
+
 _POSSIBLY_CAST_DTYPES = set([ np.dtype(t) for t in ['M8[ns]','m8[ns]','O','int8','uint8','int16','uint16','int32','uint32','int64','uint64'] ])
 _NS_DTYPE = np.dtype('M8[ns]')
 _TD_DTYPE = np.dtype('m8[ns]')
diff --git a/pandas/core/groupby.py b/pandas/core/groupby.py
index 0be5d438e5e7c..777dfaefcb9ee 100644
--- a/pandas/core/groupby.py
+++ b/pandas/core/groupby.py
@@ -1,8 +1,10 @@
 from itertools import izip
 import types
+import warnings
 import numpy as np
 
 from pandas.core.categorical import Categorical
+from pandas.core.common import PerformanceWarning
 from pandas.core.frame import DataFrame
 from pandas.core.generic import NDFrame
 from pandas.core.index import Index, MultiIndex, _ensure_index
@@ -18,7 +20,7 @@
 import pandas.lib as lib
 import pandas.algos as _algos
 import pandas.hashtable as _hash
-
+_non_agg_warning = "Function does not produce aggregated values. Will not be able to optimize and may produce unexpected results."
 _agg_doc = """Aggregate using input function or dict of {column -> function}
 
 Parameters
@@ -919,7 +921,7 @@ def _aggregate_series_pure_python(self, obj, func):
             res = func(group)
             if result is None:
                 if isinstance(res, np.ndarray) or isinstance(res, list):
-                    raise ValueError('Function does not reduce')
+                    warnings.warn(_non_agg_warning, PerformanceWarning, stacklevel=2)
                 result = np.empty(ngroups, dtype='O')
 
             counts[label] = group.shape[0]
@@ -1508,7 +1510,7 @@ def _aggregate_named(self, func, *args, **kwargs):
             group.name = name
             output = func(group, *args, **kwargs)
             if isinstance(output, np.ndarray):
-                raise Exception('Must produce aggregated value')
+                warnings.warn(_non_agg_warning, PerformanceWarning, stacklevel=2)
             result[name] = self._try_cast(output, group)
 
         return result
diff --git a/pandas/io/pytables.py b/pandas/io/pytables.py
index b1b7b80e5fd23..24d7921e04c25 100644
--- a/pandas/io/pytables.py
+++ b/pandas/io/pytables.py
@@ -19,7 +19,7 @@
 from pandas.sparse.api import SparseSeries, SparseDataFrame, SparsePanel
 from pandas.sparse.array import BlockIndex, IntIndex
 from pandas.tseries.api import PeriodIndex, DatetimeIndex
-from pandas.core.common import adjoin, isnull, is_list_like
+from pandas.core.common import adjoin, isnull, is_list_like, PerformanceWarning
 from pandas.core.algorithms import match, unique, factorize
 from pandas.core.categorical import Categorical
 from pandas.core.common import _asarray_tuplesafe, _try_sort
@@ -64,7 +64,7 @@ class AttributeConflictWarning(Warning): pass
 the [%s] attribute of the existing index is [%s] which conflicts with the new [%s],
 resetting the attribute to None
 """
-class PerformanceWarning(Warning): pass
+# for PerformanceWarning
 performance_doc = """
 your performance may suffer as PyTables will pickle object types that it cannot map
 directly to c-types [inferred_type->%s,key->%s] [items->%s]
diff --git a/pandas/io/tests/test_pytables.py b/pandas/io/tests/test_pytables.py
index 8b3d4a475d952..ff8207b11f4e3 100644
--- a/pandas/io/tests/test_pytables.py
+++ b/pandas/io/tests/test_pytables.py
@@ -10,9 +10,11 @@
 import pandas
 from pandas import (Series, DataFrame, Panel, MultiIndex, bdate_range,
                     date_range, Index)
+from pandas.core.common import PerformanceWarning
 from pandas.io.pytables import (HDFStore, get_store, Term, read_hdf, 
-                                IncompatibilityWarning, PerformanceWarning,
+                                IncompatibilityWarning,
                                 AttributeConflictWarning)
+from pandas.core.common import PerformanceWarning
 import pandas.util.testing as tm
 from pandas.tests.test_series import assert_series_equal
 from pandas.tests.test_frame import assert_frame_equal
diff --git a/pandas/tests/test_groupby.py b/pandas/tests/test_groupby.py
index f3a608b82e756..99ada1a5ee6b2 100644
--- a/pandas/tests/test_groupby.py
+++ b/pandas/tests/test_groupby.py
@@ -1,12 +1,13 @@
 import nose
 import unittest
+import warnings
 
 from datetime import datetime
 from numpy import nan
 
 from pandas import bdate_range
 from pandas.core.index import Index, MultiIndex
-from pandas.core.common import rands
+from pandas.core.common import rands, PerformanceWarning
 from pandas.core.api import Categorical, DataFrame
 from pandas.core.groupby import GroupByError, SpecificationError, DataError
 from pandas.core.series import Series
@@ -131,8 +132,11 @@ def checkit(dtype):
             self.assertEqual(agged[1], 21)
 
             # corner cases
-            self.assertRaises(Exception, grouped.aggregate, lambda x: x * 2)
-
+            with warnings.catch_warnings(record=True) as w:
+                warnings.simplefilter('always')
+                grouped.aggregate(lambda x: x * 2)
+                self.assertEqual(len(w), 1)
+                assert 'aggregate' in str(w[-1].message), "Wrong message: %r" % str(w[-1].message)
 
         for dtype in ['int64','int32','float64','float32']:
             checkit(dtype)
@@ -334,8 +338,29 @@ def test_agg_period_index(self):
 
     def test_agg_must_agg(self):
         grouped = self.df.groupby('A')['C']
-        self.assertRaises(Exception, grouped.agg, lambda x: x.describe())
-        self.assertRaises(Exception, grouped.agg, lambda x: x.index[:2])
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter('always')
+            grouped.agg(lambda x: x.describe())
+            self.assertEqual(len(w), 1)
+            assert 'aggregate' in str(w[-1].message), "Wrong message: %r" % str(w[-1].message)
+
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter('always')
+            grouped.agg(lambda x: x.index[:2])
+            self.assertEqual(len(w), 1)
+            assert 'aggregate' in str(w[-1].message), "Wrong message: %r" % str(w[-1].message)
+
+        # motivating example for #3788
+        df = DataFrame([[1, np.array([10, 20, 30])],
+                    [1, np.array([40, 50, 60])],
+                    [2, np.array([20, 30, 40])]],
+                    columns=['category', 'arraydata'])
+
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter('always')
+            df.groupby('category').agg(sum)
+            self.assertEqual(len(w), 1)
+            assert 'aggregate' in str(w[-1].message), "Wrong message: %r" % str(w[-1].message)
 
     def test_agg_ser_multi_key(self):
         ser = self.df.C