This book constitutes the refereed proceedings of the 13th International Conference on Foundations of Software Science and Computational Structures, FOSSACS 2010, held in Paphos, Cyprus, in March 2010, as part of ETAPS 2010, the European Joint Conferences on Theory and Practice of Software. The 25 revised full papers presented together with the abstract of the keynote lecture were carefully reviewed and selected from 86 full paper submissions. The papers are organized in topical sections on semantics of programming languages, probabilistic and randomised computation, concurrency and process theory, modal and temporal logics, verification, categorical and coalgebraic methods, as well as lambda calculus and types.