diff --git a/examples/variational_autoencoder.py b/examples/variational_autoencoder.py
index fd395b17f45..ba313f1c69b 100644
--- a/examples/variational_autoencoder.py
+++ b/examples/variational_autoencoder.py
@@ -45,7 +45,7 @@ def sampling(args):
 # Custom loss layer
 class CustomVariationalLayer(Layer):
     def __init__(self, **kwargs):
-        self.is_placeholder = True
+        self._is_placeholder = True
         super(CustomVariationalLayer, self).__init__(**kwargs)
 
     def vae_loss(self, x, x_decoded_mean):
diff --git a/examples/variational_autoencoder_deconv.py b/examples/variational_autoencoder_deconv.py
index f5aa1224d47..3612754c494 100644
--- a/examples/variational_autoencoder_deconv.py
+++ b/examples/variational_autoencoder_deconv.py
@@ -109,7 +109,7 @@ def sampling(args):
 # Custom loss layer
 class CustomVariationalLayer(Layer):
     def __init__(self, **kwargs):
-        self.is_placeholder = True
+        self._is_placeholder = True
         super(CustomVariationalLayer, self).__init__(**kwargs)
 
     def vae_loss(self, x, x_decoded_mean_squash):
diff --git a/keras/backend/__init__.py b/keras/backend/__init__.py
index 2e7208cf5af..00e6d52fbe8 100644
--- a/keras/backend/__init__.py
+++ b/keras/backend/__init__.py
@@ -10,6 +10,7 @@
 from .common import cast_to_floatx
 from .common import image_data_format
 from .common import set_image_data_format
+from .common import is_placeholder
 
 # Obtain Keras base dir path: either ~/.keras or /tmp.
 _keras_base_dir = os.path.expanduser('~')
diff --git a/keras/backend/cntk_backend.py b/keras/backend/cntk_backend.py
index b4161ced8e7..cfe056896a0 100644
--- a/keras/backend/cntk_backend.py
+++ b/keras/backend/cntk_backend.py
@@ -2,6 +2,7 @@
 import cntk as C
 import numpy as np
 from .common import _FLOATX, _EPSILON, image_dim_ordering, image_data_format
+from .common import is_placeholder
 from collections import defaultdict
 from contextlib import contextmanager
 import warnings
@@ -256,6 +257,7 @@ def placeholder(
         name=name)
     x._keras_shape = shape
     x._uses_learning_phase = False
+    x._is_placeholder = True
     return x
 
 
diff --git a/keras/backend/common.py b/keras/backend/common.py
index 97646067c9b..a147c7f8e1d 100644
--- a/keras/backend/common.py
+++ b/keras/backend/common.py
@@ -108,6 +108,29 @@ def cast_to_floatx(x):
     return np.asarray(x, dtype=_FLOATX)
 
 
+def is_placeholder(tensor):
+    """Returns whether a tensor is a placeholder.
+
+    # Arguments
+        tensor: A tensor instance.
+
+    # Returns
+        A boolean.
+
+    # Example
+    ```python
+        >>> from keras import backend as K
+        >>> a = K.placeholder((2, 2), sparse=False)
+        >>> print(K.is_placeholder(a))
+        True
+    ```
+    """
+    try:
+        return tensor._is_placeholder
+    except AttributeError:
+        return False
+
+
 def image_data_format():
     """Returns the default image data format convention ('channels_first' or 'channels_last').
 
diff --git a/keras/backend/tensorflow_backend.py b/keras/backend/tensorflow_backend.py
index ec6b637207d..4f109f67ea0 100644
--- a/keras/backend/tensorflow_backend.py
+++ b/keras/backend/tensorflow_backend.py
@@ -10,10 +10,12 @@
 import inspect
 import numpy as np
 import os
+from six.moves import zip_longest
 
 from .common import floatx
 from .common import _EPSILON
 from .common import image_data_format
+from .common import is_placeholder
 
 # Legacy functions
 from .common import set_image_dim_ordering
@@ -373,7 +375,7 @@ def is_keras_tensor(x):
     ```python
         >>> from keras import backend as K
         >>> np_var = numpy.array([1, 2])
-        >>> K.is_keras_tensor(np_var) # A numpy array is not a symbolic yensor.
+        >>> K.is_keras_tensor(np_var) # A numpy array is not a symbolic tensor.
         ValueError
         >>> k_var = tf.placeholder('float32', shape=(1,1))
         >>> K.is_keras_tensor(k_var) # A variable created directly from tensorflow/theano is not a Keras tensor.
@@ -431,6 +433,7 @@ def placeholder(shape=None, ndim=None, dtype=None, sparse=False, name=None):
         x = tf.placeholder(dtype, shape=shape, name=name)
     x._keras_shape = shape
     x._uses_learning_phase = False
+    x._is_placeholder = True
     return x
 
 
@@ -2222,9 +2225,12 @@ class Function(object):
         outputs: Output tensors to fetch.
         updates: Additional update ops to be run at function call.
         name: a name to help users identify what this function does.
+        fetches: Parameters forwarded to `tf.session.run(fetches)`.
+        feed_dict: Parameters forwarded to `tf.session.run(feed_dict)`.
     """
 
-    def __init__(self, inputs, outputs, updates=None, name=None, **session_kwargs):
+    def __init__(self, inputs, outputs, updates=None, name=None,
+                 fetches=None, feed_dict=None, **session_kwargs):
         updates = updates or []
         if not isinstance(inputs, (list, tuple)):
             raise TypeError('`inputs` to a TensorFlow backend function '
@@ -2235,8 +2241,11 @@ def __init__(self, inputs, outputs, updates=None, name=None, **session_kwargs):
         if not isinstance(updates, (list, tuple)):
             raise TypeError('`updates` in a TensorFlow backend function '
                             'should be a list or tuple.')
+        # self.inputs holds tf Tensor objects
         self.inputs = list(inputs)
         self.outputs = list(outputs)
+        self.fetches = fetches
+        self.feed_dict = feed_dict
         with tf.control_dependencies(self.outputs):
             updates_ops = []
             for update in updates:
@@ -2251,19 +2260,46 @@ def __init__(self, inputs, outputs, updates=None, name=None, **session_kwargs):
         self.session_kwargs = session_kwargs
 
     def __call__(self, inputs):
+        """Run the TensorFlow session
+
+        # Arguments
+            inputs: Data and values that will go to the feed_dict of Session.run()
+                if it is associated with a tensor, if it is None the tensor will
+                be added to the fetches parameter of Session.run().
+        """
         if not isinstance(inputs, (list, tuple)):
             raise TypeError('`inputs` should be a list or tuple.')
-        feed_dict = {}
-        for tensor, value in zip(self.inputs, inputs):
+        self.current_feed_dict = {} if self.feed_dict is None else self.feed_dict
+        self.feed_to_fetch_count = 0
+        self.current_fetches = self.outputs + [self.updates_op]
+        # self.inputs contains tf tensors, inputs contains feed_dict data.
+        for tensor, value in zip_longest(self.inputs, inputs, fillvalue=None):
+            if tensor is None and value is None:
+                continue
+            elif tensor is None and value is not None:
+                raise ValueError('A tensor containing None '
+                                 'was tied to value ' + str(value) +
+                                 'so Session.run() cannot execute, '
+                                 'please check your data and Model.')
+
             if is_sparse(tensor):
                 sparse_coo = value.tocoo()
                 indices = np.concatenate((np.expand_dims(sparse_coo.row, 1),
                                           np.expand_dims(sparse_coo.col, 1)), 1)
                 value = (indices, sparse_coo.data, sparse_coo.shape)
-            feed_dict[tensor] = value
+
+            if value is None and tensor is not None:
+                self.feed_to_fetch_count += 1
+                self.current_fetches.append(tensor)
+            else:
+                self.current_feed_dict[tensor] = value
+
+        if self.fetches is not None:
+            self.current_fetches += self.fetches
+
         session = get_session()
-        updated = session.run(self.outputs + [self.updates_op],
-                              feed_dict=feed_dict,
+        updated = session.run(fetches=self.current_fetches,
+                              feed_dict=self.current_feed_dict,
                               **self.session_kwargs)
         return updated[:len(self.outputs)]
 
diff --git a/keras/engine/topology.py b/keras/engine/topology.py
index ba5cbf707f5..61454f14330 100644
--- a/keras/engine/topology.py
+++ b/keras/engine/topology.py
@@ -97,6 +97,7 @@ class Node(object):
         output_shapes: list of output shape tuples.
         arguments: dictionary of keyword arguments that were passed to the
             `call` method of the layer at the call that created the node.
+        is_placeholder: Specifies if the Node represents a placeholder.
 
     `node_indices` and `tensor_indices` are basically fine-grained coordinates
     describing the origin of the `input_tensors`, verifying the following:
@@ -113,7 +114,7 @@ def __init__(self, outbound_layer,
                  input_tensors, output_tensors,
                  input_masks, output_masks,
                  input_shapes, output_shapes,
-                 arguments=None):
+                 arguments=None, is_placeholder=False):
         # Layer instance (NOT a list).
         # this is the layer that takes a list of input tensors
         # and turns them into a list of output tensors.
@@ -157,6 +158,8 @@ def __init__(self, outbound_layer,
         # Optional keyword arguments to layer's `call`.
         self.arguments = arguments
 
+        # Indicates if the node represents a placeholder variable
+        self._is_placeholder = is_placeholder
         # Add nodes to all layers involved.
         for layer in inbound_layers:
             if layer is not None:
@@ -1330,18 +1333,19 @@ def __init__(self, input_shape=None, batch_size=None,
         self.dtype = dtype
 
         if input_tensor is None:
-            self.is_placeholder = True
+            self._is_placeholder = True
             input_tensor = K.placeholder(shape=batch_input_shape,
                                          dtype=dtype,
                                          sparse=self.sparse,
                                          name=self.name)
         else:
-            self.is_placeholder = False
+            self._is_placeholder = False
             input_tensor._keras_shape = batch_input_shape
         # Create an input node to add to self.outbound_node
         # and set output_tensors' _keras_history.
         input_tensor._uses_learning_phase = False
         input_tensor._keras_history = (self, 0, 0)
+        input_tensor._is_placeholder = self._is_placeholder
         Node(self,
              inbound_layers=[],
              node_indices=[],
@@ -1351,7 +1355,8 @@ def __init__(self, input_shape=None, batch_size=None,
              input_masks=[None],
              output_masks=[None],
              input_shapes=[batch_input_shape],
-             output_shapes=[batch_input_shape])
+             output_shapes=[batch_input_shape],
+             is_placeholder=self._is_placeholder)
 
     def get_config(self):
         config = {'batch_input_shape': self.batch_input_shape,
@@ -1491,11 +1496,14 @@ def __init__(self, inputs, outputs, name=None):
             self.inputs = list(inputs)  # Tensor or list of tensors.
         else:
             self.inputs = [inputs]
+
         if isinstance(outputs, (list, tuple)):
             self.outputs = list(outputs)
         else:
             self.outputs = [outputs]
 
+        self.target_configuration = [None] * len(self.outputs)
+
         # Check for redundancy in inputs.
         if len(set(self.inputs)) != len(self.inputs):
             raise ValueError('The list of inputs passed to the model '
@@ -1535,6 +1543,8 @@ def __init__(self, inputs, outputs, name=None):
         self._output_tensor_cache = {}
         self._output_shape_cache = {}
 
+        self._input_placeholders = []
+        self._input_yield_op_tensors = []
         # User-provided arguments validation.
         for x in self.inputs:
             # Check that x is a Keras tensor.
@@ -1560,6 +1570,10 @@ def __init__(self, inputs, outputs, name=None):
                               'instantiated via `tensor = Input(shape)`.\n'
                               'The tensor that caused the issue was: ' +
                               str(x.name))
+            if K.is_placeholder(layer):
+                self._input_placeholders.append((layer, node_index, tensor_index))
+            else:
+                self._input_yield_op_tensors.append((layer, node_index, tensor_index))
         for x in self.outputs:
             if not hasattr(x, '_keras_history'):
                 cls_name = self.__class__.__name__
@@ -1621,7 +1635,7 @@ def __init__(self, inputs, outputs, name=None):
                                                    i,
                                                    layer.__class__.__name__))
             self.input_names.append(layer.name)
-            if layer.is_placeholder:
+            if K.is_placeholder(layer):
                 self._feed_input_names.append(layer.name)
                 self._feed_inputs.append(layer.input)
                 self._feed_input_shapes.append(self.inputs[i]._keras_shape)
diff --git a/keras/models.py b/keras/models.py
index 6228fc81b2a..3d9aea608c0 100644
--- a/keras/models.py
+++ b/keras/models.py
@@ -456,6 +456,7 @@ def add(self, layer):
                                  'For multi-output layers, '
                                  'use the functional API.')
 
+            layer.inbound_nodes[0].output_tensors[0]._is_placeholder = True
             self.outputs = [layer.inbound_nodes[0].output_tensors[0]]
             self.inputs = topology.get_source_inputs(self.outputs[0])
 
@@ -479,6 +480,7 @@ def add(self, layer):
                                 'should have a single output tensor. '
                                 'For multi-output layers, '
                                 'use the functional API.')
+            output_tensor._is_placeholder = True
             self.outputs = [output_tensor]
             # update self.inbound_nodes
             self.inbound_nodes[0].output_tensors = self.outputs