This book constitutes the refereed proceedings of the 9th European Symposium on Research in Computer Security, ESORICS 2004, held in Sophia Antipolis, France in September 2004. The 27 revised full papers presented were carefully reviewed and selected from 159 submissions. Among the topics addressed are access control, authorization frameworks, privacy policies, security protocols, trusted computing, anonymity, information hiding, steganography, digital signature schemes, encrypted communication, information flow control, authentication, key distribution, public key cryptography, intrusion prevention, and attack discovery.