This book constitutes the refereed post-proceedings of the 9th International Conference on Principles of Distributed Systems, OPODIS 2005, held in Pisa, Italy in December 2005. The volume presents 30 revised full papers and abstracts of 2 invited talks. The papers are organized in topical sections on nonblocking synchronization, fault-tolerant broadcast and consensus, self-stabilizing systems, peer-to-peer systems and collaborative environments, sensor networks and mobile computing, security and verification, real-time systems, and peer-to-peer systems.