This volume of the LNCS series contains the papers accepted for presentation at the Third IFIP international working conference on active networks (IWAN 2001). The workshop was held at the Sheraton University City Hotel, in Philadelphia USA, and was hosted by the University of Pennsylvania. Active networks aim to ease the introduction of network services by adding dynamic programmability to network devices such as routers, and making aspects of the programmability accessible to users. Active networks research has focused on the development and testing of active techniques, that enable dynamic programmability in a networked environment. These techniques have a wide variety of applications. At IWAN 2001 we aimed to bring together members of the various communities using active and related techniques, and provide a forum for discussion and collaboration, involving researchers, developers, and potential users. Papers presented at IWAN 2001 covered the application of active techniques to many aspects of network based communication, including active multicast, active QoS, active security, active GRIDs, and active management. In addition, there were papers on architectures, language, and API issues. Although there were only 22 submissions, the standard of the 10 accepted papers was very high. This indicated clearly a substantial amount of ongoing high quality research in active networking, despite the current unfavorable economic conditions in the telecommunications industry. The papers also demonstrated that the research is genuinely global, and justifies an international workshop of this type. We would like to thank all the authors who submitted their work.