The pioneering organizers of the ?rst UML workshop in Mulhouse, France inthe summerof1998couldhardlyhaveanticipatedthat,in littleoveradecade, theirinitiativewouldblossomintotoday'shighlysuccessfulMODELSconference series, the premier annual gathering of researchersand practitioners focusing on a very important new technical discipline: model-based software and system engineering. This expansion is, of course, a direct consequence of the growing signi?cance and success of model-based methods in practice. The conferences have contributed greatly to the heightened interest in the ?eld, attracting much young talent and leading to the gradualemergence of its correspondingscienti?c and engineering foundations. The proceedings from the MODELS conferences are one of the primary references for anyone interested in a more substantive study of the domain. The 12th conference took place in Denver in the USA, October 4-9, 2009 along with numerous satellite workshops and tutorials, as well as several other related scienti?c gatherings. The conference was exceptionally fortunate to have three eminent, invited keynote speakers from industry: Stephen Mellor, Larry Constantine, and Grady Booch.