Book excerpt: A software agent has a spectrum of definitions. At one end of the scale are relatively simple, client-based software applications that can assist users in performing mundane tasks such as sorting e-mail or downloading web pages. This class of agents is often referred to as “personal assistant” agents. At the other end of the scale is the concept of sophisticated software entities possessing artificial intelligence that autonomously travel through a network environment and make complex decisions on a user's behalf.In telecommunications, the definition lies somewhere between those two extremes. This classification of mobile agents, although not strictly adhering to the definition of “intelligent agents” originally proposed by the artificial intelligence community, is generally acknowledged to be a useful categorization and has started to gain widespread acceptance. We therefore define a mobile agent as a program that acts on behalf of a user or another program and is able to migrate from host to host on a network under its own control. The agent chooses when and where it will migrate and may interrupt its own execution and continue elsewhere on the network. The agent returns results and messages in an asynchronous fashion.This volume discusses the emerging field of mobile software agents and their applications to the area of telecommunications, such as active networks, e-commerce, the Internet, interactive QoS, network management, and feature interactions. It addresses the needs of a wide audience, including researchers, software agent systems and telecommunication applications designers, and users of software agents.