This book constitutes the refereed proceedings of the international Joint Modular Languages Conference, JMLC 2003, held in Klagenfurt, Austria in August 2003.The 17 revised full papers and 10 revised short papers presented together with 5 invited contributions were carefully reviewed and selected from 47 submissions. The papers are organized in topical sections on architectural concepts and education, component architectures, language concepts, frameworks and design principles, compilers and tools, and formal aspects and reflective programming.