This book constitutes the refereed proceedings of the 7th International Conference on Foundations of Software Science and Computation Structures, FOSSACS 2004, held in Barcelona, Spain in March/April 2004. The 34 revised full papers presented together with the abstracts of 2 invited talks were carefully reviewed and selected from over 130 submissions. Among the topics addressed are lambda calculus, cryptographic protocol analysis, graphs and grammar systems, decision theory, bisimulation, rewriting, normalization, specification, verification, process calculi, mobile code, automata, program semantics, dynamic logics, timed languages, security analysis, information-theoretical aspects.