This book constitutes the thoroughly refereed post-proceedings of the Third International Workshop on Peer-to-Peer Systems, IPTPS 2004, held in La Jolla, CA, USA, in February 2004. The 27 revised full papers presented together with a workshop report were carefully selected during two rounds of reviewing and improvement from 145 submissions. All in all, the papers document the state of the art in peer-to-peer computing research; they are organized in topical sections on networking, routing, load balancing and scheduling, applications, security, and miscellaneous.