diff --git a/tailbone/diffs.py b/tailbone/diffs.py
index d4031b1f..d57aa9ac 100644
--- a/tailbone/diffs.py
+++ b/tailbone/diffs.py
@@ -35,8 +35,29 @@ class Diff(object):
     Core diff class.  In sore need of documentation.
     """
 
-    def __init__(self, old_data, new_data, columns=None, fields=None, render_field=None, render_value=None, monospace=False,
-                 extra_row_attrs=None):
+    def __init__(self, old_data, new_data, columns=None, fields=None,
+                 render_field=None, render_value=None,
+                 monospace=False, extra_row_attrs=None):
+        """
+        Constructor.  You must provide the old and new data sets, and
+        the set of relevant fields as well, if they cannot be easily
+        introspected.
+
+        :param old_data: Dict of "old" data values.
+
+        :param new_data: Dict of "old" data values.
+
+        :param fields: Sequence of relevant field names.  Note that
+           both data dicts are expected to have keys which match these
+           field names.  If you do not specify the fields then they
+           will (hopefully) be introspected from the old or new data
+           sets; however this will not work if they are both empty.
+
+        :param monospace: If true, this flag will cause the value
+           columns to be rendered in monospace font.  This is assumed
+           to be helpful when comparing "raw" data values which are
+           shown as e.g. ``repr(val)``.
+        """
         self.old_data = old_data
         self.new_data = new_data
         self.columns = columns or ["field name", "old value", "new value"]