Active networks represent a significant step in the evolution of packet-switched networks, from traditional packet-forwarding engines to more general functionality supporting dynamic control and modification of network behavior. However, the phrase ^\active network^] means different things to different people. This survey introduces a model and nomenclature for talking about active networks, describes some possible approaches in terms of that nomenclature, and presents various aspects of the architecture being developed in the DARPA-funded active networks program. Also, a snapshot of the current research issues and activities of different institutions is provided. Potential applications of active networks are highlighted, along with some of the challenges that must be overcome to make them reality.