Description:This book constitutes the refereed proceedings of the 12th International SPIN workshop on Model Checking Software, SPIN 2005, held in San Francisco, USA in August 2005.The 15 revised full papers presented were carefully reviewed and selected from 45 submissions; in addition there are 4 tool presentation papers selected from 6 submissions. The papers are organized in topical sections on state representation and abstraction, dealing with concurrency, dealing with complex data, checking temporal properties, and checking security and real-time properties.