This book constitutes the refereed proceedings of the 11th International SPIN workshop on Model Checking Software, SPIN 2004, held in Barcelona, Spain, in April 2004. The 19 revised full papers presented together with the abstracts of an invited talk and 2 tutorials were carefully reviewed and selected from 48 submissions. The papers are organized in topical sections on heuristics and probabilities, improvements of SPIN, validation of timed systems, tool presentations, abstraction and symbolic methods, and applications.