This book constitutes the refereed proceedings of the 12th International Conference on Algebraic Methodology and Software Technology, AMAST 2008, held in Urbana, IL, USA, in July 2008. The 28 revised full papers presented together with 3 invited talks were carefully reviewed and selected from 58 submissions. Among the topics covered are all current issues in formal methods related to algebraic and logical foundations, software technology, and to programming methodology including concurrent and reactive systems, evolutionary software/adaptive systems, logic and functional programming, object paradigms, constraint programming and concurrency, program verification and transformation, programming calculi, specification languages and tools, formal specification and development case studies, logic, category theory, relation algebra, computational algebra, algebraic foundations for languages and systems, coinduction, theorem proving and logical frameworks for reasoning, logics of programs, as well as algebra and coalgebra.