This book constitutes the thoroughly refereed post-proceedings of the 5th International Workshop on Formal Approaches to Software Testing, FATES 2005, held in Edinburgh, UK, in July 2005 in conjunction with CAV 2005. The book presents 13 revised full papers together with 1 work-in-progress paper. These address formal approaches to testing and use techniques from areas like theorem proving, model checking, constraint resolution, program analysis, abstract interpretation, Markov chains, and various others.