Interaction is an emerging paradigm of models of computation that re?ects the shift in technology from mainframes to networks of intelligent agents, from number-crunching to embedded systems to graphical user interfaces, andfromprocedure-orientedto object-baseddistributedsystems.Interacti- based models di?er from the Turing-machine-basedalgorithmic models of the 1960s in interesting and useful ways: ProblemSolving:Models ofinteractioncapturethenotionofperforminga task or providing a service,ratherthan algorithmicallyproducing outputs from inputs. Observable Behavior: In models of interaction, a computing component is modeled not as a functional transformation from input to output, but rather in terms of observable behavior consisting of interaction steps. For example, interactions may consist of interleaved inputs and outputs m- eled by dynamic streams; future input values can depend on past output values. Environments: In models of interaction, the world or environment of the computationis partofthemodelandplaysanactivepartinthecompu- tion by dynamically supplying the computational system, or agent, with inputs, and consuming the output values the system produces. The en- ronment cannot be assumed to be static or even e?ectively computable; for example, it may include humans or other real-world elements. Concurrency: In models of interaction, computation may be concurrent; a computing agent can compute in parallel with its environment and with other agents. The interaction paradigm provides a new conceptualization of compu- tional phenomena that emphasizes interaction rather than algorithms. C- current, distributed, reactive, embedded, component-oriented, agent-oriented andservice-orientedsystemsallexploitinteractionasafundamentalparadigm.