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...