This book constitutes the thoroughly refereed post-proceedings of the 12th International Workshop on Logic Based Program Synthesis and Transformation, LOPSTR 2002, held in Madrid, Spain in September 2002. The 15 revised full papers presented together with 7 abstracts were carefully selected during two rounds of reviewing and revision from 40 submissions. The papers are organized in topical sections on debugging and types, tabling and constraints, abstract interpretation, program refinement, verification, partial evaluation, and rewriting and object-oriented development.