--- a/toolkit/components/telemetry/histogram_tools.py
+++ b/toolkit/components/telemetry/histogram_tools.py
@@ -4,16 +4,20 @@
 
 import collections
 import json
 import math
 import os
 import re
 import sys
 
+# Constants.
+MAX_LABEL_LENGTH = 20
+MAX_LABEL_COUNT = 100
+
 # histogram_tools.py is used by scripts from a mozilla-central build tree
 # and also by outside consumers, such as the telemetry server.  We need
 # to ensure that importing things works in both contexts.  Therefore,
 # unconditionally importing things that are local to the build tree, such
 # as buildconfig, is a no-no.
 try:
     import buildconfig
 
@@ -227,18 +231,19 @@ associated with the histogram.  Returns 
         if not self._strict_type_checks:
             table['exponential'].append('extended_statistics_ok')
 
         table_dispatch(definition['kind'], table,
                        lambda allowed_keys: Histogram.check_keys(name, definition, allowed_keys))
 
         self.check_name(name)
         self.check_field_types(name, definition)
+        self.check_whitelistable_fields(name, definition)
         self.check_expiration(name, definition)
-        self.check_whitelistable_fields(name, definition)
+        self.check_label_values(name, definition)
 
     def check_name(self, name):
         if '#' in name:
             raise ValueError, '"#" not permitted for %s' % (name)
 
     def check_expiration(self, name, definition):
         expiration = definition.get('expires_in_version')
 
@@ -247,16 +252,38 @@ associated with the histogram.  Returns 
 
         if re.match(r'^[1-9][0-9]*$', expiration):
             expiration = expiration + ".0a1"
         elif re.match(r'^[1-9][0-9]*\.0$', expiration):
             expiration = expiration + "a1"
 
         definition['expires_in_version'] = expiration
 
+    def check_label_values(self, name, definition):
+        labels = definition.get('labels')
+        if not labels:
+            return
+
+        invalid = filter(lambda l: len(l) > MAX_LABEL_LENGTH, labels)
+        if len(invalid) > 0:
+            raise ValueError, 'Label values for %s exceed length limit of %d: %s' % \
+                              (name, MAX_LABEL_LENGTH, ', '.join(invalid))
+
+        if len(labels) > MAX_LABEL_COUNT:
+            raise ValueError, 'Label count for %s exceeds limit of %d' % \
+                              (name, MAX_LABEL_COUNT)
+
+        # To make it easier to generate C++ identifiers from this etc., we restrict
+        # the label values to a strict pattern.
+        pattern = '^[a-z][a-z0-9_]+[a-z0-9]$'
+        invalid = filter(lambda l: not re.match(pattern, l, re.IGNORECASE), labels)
+        if len(invalid) > 0:
+            raise ValueError, 'Label values for %s are not matching pattern "%s": %s' % \
+                              (name, pattern, ', '.join(invalid))
+
     # Check for the presence of fields that old histograms are whitelisted for.
     def check_whitelistable_fields(self, name, definition):
         # Use counters don't have any mechanism to add the fields checked here,
         # so skip the check for them.
         if self._is_use_counter:
             return
 
         # In the pipeline we don't have whitelists available.