diff --git a/core/google/cloud/iterator.py b/core/google/cloud/iterator.py
index ac1323a35e19..06242e57597d 100644
--- a/core/google/cloud/iterator.py
+++ b/core/google/cloud/iterator.py
@@ -246,10 +246,11 @@ def __init__(self, client, path, item_to_value,
                  items_key=DEFAULT_ITEMS_KEY,
                  page_token=None, max_results=None, extra_params=None,
                  page_start=_do_nothing_page_start):
+        self._started = False
         self.client = client
         self.path = path
-        self._items_key = items_key
         self._item_to_value = item_to_value
+        self._items_key = items_key
         self.max_results = max_results
         self.extra_params = extra_params
         self._page_start = page_start
@@ -288,7 +289,15 @@ def page(self):
         return self._page
 
     def __iter__(self):
-        """The :class:`Iterator` is an iterator."""
+        """The :class:`Iterator` is an iterator.
+
+        :rtype: :class:`Iterator`
+        :returns: Current instance.
+        :raises ValueError: If the iterator has already been started.
+        """
+        if self._started:
+            raise ValueError('Iterator has already started', self)
+        self._started = True
         return self
 
     def update_page(self, require_empty=True):
@@ -385,10 +394,3 @@ def _get_next_page_response(self):
         self.next_page_token = response.get('nextPageToken')
 
         return response
-
-    def reset(self):
-        """Resets the iterator to the beginning."""
-        self.page_number = 0
-        self.next_page_token = None
-        self.num_results = 0
-        self._page = _UNSET
diff --git a/core/unit_tests/test_iterator.py b/core/unit_tests/test_iterator.py
index 81a3738ffc1f..e16edaaa00fa 100644
--- a/core/unit_tests/test_iterator.py
+++ b/core/unit_tests/test_iterator.py
@@ -101,14 +101,26 @@ def _makeOne(self, *args, **kw):
         return self._getTargetClass()(*args, **kw)
 
     def test_constructor(self):
+        from google.cloud.iterator import _do_nothing_page_start
+        from google.cloud.iterator import _UNSET
+
         connection = _Connection()
         client = _Client(connection)
         path = '/foo'
         iterator = self._makeOne(client, path, None)
+        self.assertFalse(iterator._started)
         self.assertIs(iterator.client, client)
         self.assertEqual(iterator.path, path)
+        self.assertIsNone(iterator._item_to_value)
+        self.assertEqual(iterator._items_key, 'items')
+        self.assertIsNone(iterator.max_results)
+        self.assertEqual(iterator.extra_params, {})
+        self.assertIs(iterator._page_start, _do_nothing_page_start)
+        # Changing attributes.
         self.assertEqual(iterator.page_number, 0)
         self.assertIsNone(iterator.next_page_token)
+        self.assertEqual(iterator.num_results, 0)
+        self.assertIs(iterator._page, _UNSET)
 
     def test_constructor_w_extra_param_collision(self):
         connection = _Connection()
@@ -193,6 +205,13 @@ def test___iter__(self):
         iterator = self._makeOne(None, None, None)
         self.assertIs(iter(iterator), iterator)
 
+    def test___iter___started(self):
+        iterator = self._makeOne(None, None, None)
+        iter_obj = iter(iterator)
+        self.assertIs(iter_obj, iterator)
+        with self.assertRaises(ValueError):
+            iter(iterator)
+
     def test_iterate(self):
         import six
 
@@ -336,23 +355,6 @@ def test__get_next_page_response_new_no_token_in_response(self):
         self.assertEqual(kw['path'], path)
         self.assertEqual(kw['query_params'], {})
 
-    def test_reset(self):
-        from google.cloud.iterator import _UNSET
-
-        connection = _Connection()
-        client = _Client(connection)
-        path = '/foo'
-        token = 'token'
-        iterator = self._makeOne(client, path, None)
-        iterator.page_number = 1
-        iterator.next_page_token = token
-        iterator._page = object()
-        iterator.reset()
-        self.assertEqual(iterator.page_number, 0)
-        self.assertEqual(iterator.num_results, 0)
-        self.assertIsNone(iterator.next_page_token)
-        self.assertIs(iterator._page, _UNSET)
-
 
 class _Connection(object):