This book provides a thorough and comprehensive introduction to computer security. It covers both the theoretical foundations and practical aspects of secure systems. The theory demonstrates what is possible; the practical shows what is feasible. The book also discusses policy, because policy ultimately defines what is and is not "secure." Throughout, the well-known author reveals the multiplex interrelationships among the many subdisciplines of computer security, and provides numerous examples to illustrate the principles and techniques discussed. This long-awaited book has been thoroughly class-tested at both the advanced undergraduate level, with an emphasis on its more applied coverage, and at the introductory graduate level, where theory and practice are covered more equally.