This book constitutes the thoroughly refereed post-proceedings of the 9th International Workshop on Security Protocols held in Cambridge, UK in April 2001. The 13 revised full papers presented together with transcriptions of the discussions following the presentations have gone through two rounds of reviewing, revision, and selection. Also included are abstracts and transcriptions of invited presentations and topically focused discussions. Among the topics addressed are mobile computing and security, denial of service, authentication, Internet protocols, timing attacks, PIM security, security engineering, non-repudiation, trust management, access control policies, and Bluetooth security.