This book constitutes the refereed proceedings of the 5th International Passive and Active Measurement Workshop, PAM 2004, held in Antibes Juan-les-Pins, France in April 2004.The 29 revised full papers presented were carefully reviewed and selected from 184 submissions. The papers are organized in topical sections on P2P and overlay, network optimization, traffic analysis, protocol and system measurement, tools, miscellaneous, network measurement, and BGP and routing.