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and re?nement; transition systems; and type systems and type theory.We received 96 submissions, of which 2 were withdrawn.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.

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.

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.