Spin—an efficient verification system for distributed software systems. Spin is a popular open-source software tool, used by thousands of people worldwide, that can be used for the ”’formal verification of distributed software systems”’. The tool was developed at Bell Labs in the original Unix group of the Computing Sciences Research Center, starting in 1980. The software has been available freely since 1991, and continues to evolve to keep pace with new developments in the field. In April 2002 the tool was awarded the prestigious System Software Award for 2001 by the ACM.