A very transparent, explicit model of chemical reactions is provided by graph grammars, where molecules are encoded as graphs and reactions are defined by transformation rules that rewrite local patterns, i.e., bonds. Such a system is implemented in MØD. Transformation rules, e.g. for well-known named reactions, are not challenging to write manually; for instance, Synthia (or Chematica 8 ) successfully applies this approach in its complex synthesis planning, utilizing a database of approximately 70,000 expert-encoded rules. This level of effort, however, is unlikely to scale to the entirety of present-day chemical knowledge, and cannot keep pace with the continuing exponential increase in the number of reported reactions. Hence, automatic methods to mine comprehensive data repositories for transformation rules are urgently needed.