The mission of JMLC is to explore the concepts of well-structured rogramming languages and software, and those of teaching good design and programming style. A special interest is the construction of large and distributed software systems. JMLC - in the future it may be renamed to PMLC (Programming Methodology and Languages) - has already a long tradition; earlier confer- ces were held in 1987 (Bled, Slovenia), 1990 (Loughborough, UK), 1994 (Ulm, Germany), 1997 (Linz, Austria) and 2000 (Zuric ¨ h, Switzerland). JMLC 2003 attracted 47 papers from 10 countries (Australia, Austria, Czech Republic, Germany, Hungary, Italy, Switzerland, Russia, UK, USA). Each paper was reviewed by three representatives of the international program committee. Seventeen papers were accepted as full and 10 further papers as short contri- tions. In a second, simpli?ed reviewing process the editors of this proceedings checked that all authors had accounted for the detailed comments of the rev- wers. In addition to the regular program, JMLC 2003 invited ?ve distinguished speakers: Niklaus Wirth (ETH Zuric ¨ h), Michael Franz (UCI, Irvine), Jayadev Misra (UT, Austin), C.A.R. Hoare (Microsoft Research and Oxford University) and Jim Miller (Microsoft Corporation), the latter two in common with the co-located EuroPar 2003 conference. JMLC 2003 invited four tutorials: On.NET (Hanspeter M¨ ossenb¨ ock, Wo- gang Beer, Dietrich Birngruber and Albrecht W¨ oß, University Linz and Tech- Talk);C# (JudithBishop,UniversityofPretoriaandNigelHorspool,University of Victoria); and Design by Contract and the Ei?el Method and Trusted Com- nentsandAttemptsatProofs (BertrandMeyer,ETHZuric ¨ handEi?elSoftware).