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,...