This book constitutes the thoroughly refereed proceedings of the 21st International Symposium on Logic-Based Program Synthesis and Transformation, LOPSTR 2011, held in Odense, Denmark in July 2011. The 6 revised full papers presented together with 8 additional papers were carefully reviewed and selected from 28 submissions. Among the topics covered are specification, synthesis, verification, analysis, optimization, specialization, security, certification, applications and tools, program/model manipulation, and transformation techniques for any programming language paradigm.