This book constitutes the refereed proceedings of the 12th European Symposium on Programming, ESOP 2003, held in Warsaw, Poland, in April 2003. The 25 revised full papers presented together with two invited papers were carefully reviewed and selected from 99 submissions. Among the topics addressed are programming paradigms and their integration, program semantics, calculi of computation, security, advanced type systems, program analysis, program transformation, and practical algorithms based on theoretical developments.