This book constitutes the refereed proceedings of the 21st Annual Working Conference on Data and Applications Security held in Redondo Beach, CA, USA in July 2007. The 18 revised full papers and 2 revised short papers presented were carefully reviewed and selected from 44 submissions. The papers are organized in topical sections on secure query evaluation, location-based security/mobile security, distributed security issues, cryptographic-based security, temporal access control and usage control, as well as system security issues