This book constitutes the refereed proceedings of the 12th International Conference on Theory and Applications of Satisfiability Testing, SAT 2009, held in Swansea, UK, in June/July 2009. The 34 revised full papers presented together with 11 revised short papers and 2 invited talks were carefully selected from 86 submissions. The papers are organized in topical sections on applications of SAT, complexity theory, structures for SAT, resolution and SAT, translations to CNF, techniques for conflict-driven SAT Solvers, solving SAT by local search, hybrid SAT solvers, automatic adaption of SAT solvers, stochastic approaches to SAT solving, QBFs and their representations, optimization algorithms, distributed and parallel solving.