The present volume contains the proceedings of the international conference Foundations of Software Science and Computation Structures(FOSSACS)2003, held in Warsaw, Poland, April 7-9, 2003. FOSSACS is an event of the Joint European Conferences on Theory and Practice of Software (ETAPS). The pre- ous ?ve FOSSACS conferences took place in Lisbon (1998), Amsterdam (1999), Berlin (2000), Genoa (2001), and Grenoble (2002). FOSSACS presents original papers on foundational researchwith a clear s- ni?cancetosoftwarescience.TheProgramCommitteeinvitedpapersontheories and methodsto supportthe analysis,integration,synthesis,transformation,and veri?cation of programs and software systems. We identi?ed the following t- ics, in particular: algebraic models; automata and language theory; behavioural equivalences; categorical models; computation processes over discrete and c- tinuous data; computation structures; logics of programs; modal, spatial, and temporallogics;modelsof concurrent,reactive,distributed, and mobile systems; processalgebrasandcalculi;semanticsofprogramminglanguages;softwaresp- i?cation and re?nement; transition systems; and type systems and type theory. We received 96 submissions, of which 2 were withdrawn. This proceedings consists of 27 papers. The ?rst-A Game Semantics for Generic Polymorphism-accompanies the invited lecture by Samson Abramsky, University of Oxford. The remaining 26 were selected for publication by the Program Committee during a week-long electronic discussion. I sincerely thank all the authors of papers submitted to FOSSACS 2003; the number and the quality of papers were exceptionally high this year. Moreover, I would like to thank all the members of the ProgramCommittee for the excellent job they did during a rather demanding selection process, and to thank all our subreferees for their invaluable contributions to this process.