This book constitutes the refereed proceedings of the 12th International Conference on Model Driven Engineering Languages and Systems, MoDELS 2009, held in Denver, CO, USA, during October 4-9, 2009. The 45 revised full papers and 13 short papers presented were carefully reviewed and selected from 248 submissions. The book also contains three keynote speeches and contributions to workshops, symposia, tutorials and panels at the conference. The papers are organized in topical sections on (meta-)model Modeling and Management, quantitative modeling with UML, model transformations and contraints, model management, UML in practice and quality assurance, formalization of model transformations, scenario modeling, business application development, model synchronisation and change propagation, language specification and annotation, domain-specific languages, model-based analysis, model (de-)composition and abstraction, distributed software development, service and business process integration, genericity and constraints, variability management, and model transformation engineering.