Active networks research has been ongoing now for around five years since the seminal paper by Tennenhouse. A number of workshops and conferences have now sprung up, and several programs of work are in place around the world building prototype active network middleware, testbeds, services and applications. The aim of this special issue is to gather together some of the output from these projects into a single archival volume for reference by researchers in the next generation of active networks work. To this end, we were particularly interested in receiving submissions with practical results, but also interested in theoretical and architectural papers.