This book constitutes the refereed proceedings of the 9th International Conference on Foundations of Software Science and Computation Structures, FOSSACS 2006, held in Vienna, Austria in March 2006 as part of ETAPS.

The 28 revised full papers presented together with 1 invited paper were carefully reviewed and selected from 107 submissions. The papers are organized in topical sections on mobile processes, software science, distributed computation, categorical models, real time and hybrid systems, process calculi, automata and logic, domains, lambda calculus, types, and security.