This book constitutes the refereed proceedings of the IFIP-TC6 4th International Working Conference on Active Networks, IWAN 2002, held in Zurich, Switzerland, in December 2002. The 20 revised full papers presented were carefully reviewed and selected from 53 submissions. Among the topics addressed are router architectures, reconfigurable systems, NodeOS, service deployment, active network services, active network queries, network management agents, active network performance, mobile communications, programmable networks, network execution environments, active network architecture, group communication, peer-to-peer networks, and interaction detection.