This article describes the mobile-agent paradigm, which is becoming increasingly popular for network-centric programming, and compare it with earlier paradigms for distributed computing from which it has evolved. The design of mobile agent systems requires that several system-level issues be resolved, such as the provision of code mobility, object naming, portability, scalability, and a range of security issues that go hand-in-hand with mobile code. Agent programming requires suitable languages and programming models that can support code mobility and runtime systems that provide some fundamental primitives for the creation, migration, and management of agents. The authors discuss these requirements and describe several mobile agent systems that illustrate different approaches designers have taken to address the problems.