This volume of the LNCS series contains the proceedings of the 5th Internat- nal Working Conference on Active Networks (IWAN 2003) held in the ancient cultural city of Kyoto, Japan. This year we received 73 submissions. The increasing number indicates that Active Networks continues to be an attractive ?eld of research. Through - reful reviewing and discussion, our program committee decided to fully accept 21 papers. Three papers were conditionally accepted, and were included after shepherding by members of the technical program committee. This volume thus includes these 24 papers which were presented at IWAN 2003. Additional papers were presented in a poster session at the conference. The best paper award went to Kenneth L. Calvert, James N. Gri?oen, - jati Imam, and Jiangbo Li (University of Kentucky) for “Challenges in Imp- menting an ESP Service,” which begins these proceedings and which began the papers in the High Performance & Network Processors session. Papers in these proceedings are organized into seven sessions: High-Level Active Network - plications, Low-Level Active Network Applications, Self-Organization of Active Services, Management in Active Networks, Experiences with Service Engin- ring for Active Networks, and Selected Topics in Active Networks, ranging from risk management to context-aware handover and peer-to-peer communications.