diff --git a/mercurial/cmdutil.py b/mercurial/cmdutil.py
--- a/mercurial/cmdutil.py
+++ b/mercurial/cmdutil.py
@@ -8,6 +8,7 @@
 from __future__ import absolute_import
 
 import errno
+import itertools
 import os
 import re
 import tempfile
@@ -1452,6 +1453,7 @@ class changeset_templater(changeset_prin
             self.t = formatter.maketemplater(ui, 'changeset', tmpl,
                                              cache=defaulttempl)
 
+        self._counter = itertools.count()
         self.cache = {}
 
         # find correct templates for current mode
@@ -1490,6 +1492,7 @@ class changeset_templater(changeset_prin
         props['ctx'] = ctx
         props['repo'] = self.repo
         props['ui'] = self.repo.ui
+        props['index'] = next(self._counter)
         props['revcache'] = {'copies': copies}
         props['cache'] = self.cache
 
diff --git a/mercurial/formatter.py b/mercurial/formatter.py
--- a/mercurial/formatter.py
+++ b/mercurial/formatter.py
@@ -103,6 +103,7 @@ baz: foo, bar
 
 from __future__ import absolute_import
 
+import itertools
 import os
 
 from .i18n import _
@@ -338,6 +339,7 @@ class templateformatter(baseformatter):
         self._topic = topic
         self._t = gettemplater(ui, topic, opts.get('template', ''),
                                cache=templatekw.defaulttempl)
+        self._counter = itertools.count()
         self._cache = {}  # for templatekw/funcs to store reusable data
     def context(self, **ctxs):
         '''insert context objects to be used to render template keywords'''
@@ -350,6 +352,7 @@ class templateformatter(baseformatter):
         props = {}
         if 'ctx' in self._item:
             props.update(templatekw.keywords)
+        props['index'] = next(self._counter)
         # explicitly-defined fields precede templatekw
         props.update(self._item)
         if 'ctx' in self._item:
diff --git a/mercurial/templatekw.py b/mercurial/templatekw.py
--- a/mercurial/templatekw.py
+++ b/mercurial/templatekw.py
@@ -7,6 +7,7 @@
 
 from __future__ import absolute_import
 
+from .i18n import _
 from .node import hex, nullid
 from . import (
     encoding,
@@ -422,6 +423,12 @@ def showgraphnode(repo, ctx, **args):
     else:
         return 'o'
 
+@templatekeyword('index')
+def showindex(**args):
+    """Integer. The current iteration of the loop. (0 indexed)"""
+    # just hosts documentation; should be overridden by template mapping
+    raise error.Abort(_("can't use index in this context"))
+
 @templatekeyword('latesttag')
 def showlatesttag(**args):
     """List of strings. The global tags on the most recent globally
diff --git a/mercurial/templater.py b/mercurial/templater.py
--- a/mercurial/templater.py
+++ b/mercurial/templater.py
@@ -411,8 +411,9 @@ def runmap(context, mapping, data):
             else:
                 raise error.ParseError(_("%r is not iterable") % d)
 
-    for v in diter:
+    for i, v in enumerate(diter):
         lm = mapping.copy()
+        lm['index'] = i
         if isinstance(v, dict):
             lm.update(v)
             lm['originalnode'] = mapping.get('node')
diff --git a/tests/test-command-template.t b/tests/test-command-template.t
--- a/tests/test-command-template.t
+++ b/tests/test-command-template.t
@@ -2683,6 +2683,16 @@ Pass generator object created by templat
   $ hg log -l 1 --template '{if(author, author)|user}\n'
   test
 
+Test index keyword:
+
+  $ hg log -l 2 -T '{index + 10}{files % " {index}:{file}"}\n'
+  10 0:a 1:b 2:fifth 3:fourth 4:third
+  11 0:a
+
+  $ hg branches -T '{index} {branch}\n'
+  0 default
+  1 foo
+
 Test diff function:
 
   $ hg diff -c 8