This book constitutes the refereed proceedings of the 21st International Conference on Computer Aided Verification, CAV 2009, held in Grenoble, France, in June/July 2009. The 36 revised full papers presented together with 16 tool papers and 4 invited talks and 4 invited tutorials were carefully reviewed and selected from 135 regular paper and 34 tool paper submissions. The papers are dedicated to the advancement of the theory and practice of computer-aided formal analysis methods for hardware and software systems; their scope ranges from theoretical results to concrete applications, with an emphasis on practical verification tools and the underlying algorithms and techniques.