diff --git a/doc/MANUAL.md b/doc/MANUAL.md
index af42cbb..4bdeb2b 100644
--- a/doc/MANUAL.md
+++ b/doc/MANUAL.md
@@ -183,6 +183,9 @@ Structurally, they are made up of the following elements:
         - **`style`**: A string of inline CSS which will be applied to the field.
           **Optional**, **String**.
 
+        - **`classes`**: A string of optional CSS classes to add to this field.
+          **Optional**, **String**.
+
         - **`...`**: Other options, which depend on the type of field.  For
           more information, see [Field types](#field_types). **Optional**.
 
diff --git a/examples/customize/customize.json b/examples/customize/customize.json
index 4dfeffc..240487d 100644
--- a/examples/customize/customize.json
+++ b/examples/customize/customize.json
@@ -12,7 +12,8 @@
                     "name": "background",
                     "title": "Different background color",
                     "type": "string",
-                    "style": "background-color: #C0FFC0;"
+                    "style": "background-color: #C0FFC0;",
+                    "classes": "foo bar"
                 }
             ]
         }
diff --git a/src/webapp.py b/src/webapp.py
index d1371df..2342561 100644
--- a/src/webapp.py
+++ b/src/webapp.py
@@ -373,6 +373,8 @@ class ScriptFormWebApp(WebAppHandler):
             if field.get('required', None):
                 params['classes'].append('required')
 
+            params['classes'].extend(field.get('classes', '').split())
+
             params["style"] = field.get("style", "")
 
             # Get field-specific parameters