This book constitutes the refereed proceedings of the 20th International Symposium on Model Checking Software, SPIN 2013, held in Stony Brook, NY, USA, in July 2013. The 18 regular papers, 2 tool demonstration papers, and 2 invited papers were carefully reviewed and selected from 40 submissions. The traditional focus of SPIN has been on explicit-state model checking techniques, as implemented in SPIN and other related tools. While such techniques are still of key interest to the workshop, its scope has broadened over recent years to include techniques for the verification and formal testing of software systems in general.