The pioneering organizers of the first UML workshop in Mulhouse, France inthe summer of 1998 could hardly have anticipated that, in littleoveradecade, their initiative would blossom into today’s highly successful MODELS conference 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 significance and success of model-based methods in practice. The conferences have contributed greatly to the heightened interest in the field, attracting much young talent and leading to the gradualemergence of its correspondingscientific 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 scientific gatherings. The conference was exceptionally fortunate to have three eminent, invited keynote speakers from industry: Stephen Mellor, Larry Constantine, and Grady Booch.