A virtual organization (VO) is a dynamic collection of entities (individuals, enterprises, and information resources) collaborating on some computational activity. VOs are an emerging means to model, enact, and manage largescale service computations.

VOs consist of autonomous, heterogeneous members, often exhibiting complex behaviors. Thus VOs are a natural match for policy-based approaches. Traditional policybased frameworks emphasize reactive behaviors, wherein an external request causes a policy engine to compute a response. However, business service settings require richer policies and call for proactive behaviors. A business not only must respond to explicit requests, but also monitor its environment, collate events, and potentially act in anticipation of events in order to ensure that its policies are satisfied. Autonomous, heterogeneous, proactive entities are best modeled as agents and, therefore, VOs are best understood as multiagent systems.

Our main contributions are (1) a proactive multiagent policy-based architecture, (2) a hierarchical model of policy monitoring, compliance checking, and enforcement for VOs, and (3) a formalization of VOs. We evaluate our approach using a real business service scenario.