The International Conference "Foundations of Software Science and Compu- tion Structures" (FOSSACS) is a constituent of the "Joint European Conferences on Theoryand Practice of Software" (ETAPS). The present volume contains the contributions to FOSSACS'99, the second conference in this series, which took place in Amsterdam. As formulated in the call for papers, FOSSACS focusses on "papers which o'er progress in foundational research with a clear signi'cance...