This book constitutes the proceedings of the 18th International Conference on Foundations of Software Science and Computation Structures, FOSSACS 2015, held in London, UK, in April 2015, as part of the European Joint Conferences on Theory and Practice of Software, ETAPS 2015. The 28 full papers presented in this volume were carefully reviewed and selected from 93 submissions. They are organized in topical sections named: semantics of programming languages; categorical models and logics; concurrent, probabilistic and timed systems; automata, games, verification; logical aspects of computational complexity; and type theory, proof theory and implicit computational complexity. The book also contains one full paper invited talk.