This book constitutes the refereed proceedings of the 12th International Conference on Fundamental Approaches to Software Engineering, FASE 2009, held in York, UK, in March 2009, as part of ETAPS 2009, the European Joint Conferences on Theory and Practice of Software. The 30 revised full papers presented together with 2 tool demonstrations were carefully reviewed and selected from 123 regluar and 9 tool paper submissions. The topics addressed are model-driven development, synthesis and adaptation, modeling, testing and debugging, model analysis, patterns, security, queries and error handling, and tools (demos) and program analysis.