The increasing popularity of mobile computing devices, coupled with rapid advances in wireless networking technologies, have created the infrastructure needed to support the anywhere-anytime computing paradigm. Middleware systems have started to appear that aim at facilitating coordination among these devices, without the user even thinking about it, thus receding technology into the background. However, faced with overwhelming choice, additional support is required for applications to decide

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... o can be trusted among this plethora of interacting peers. In this paper we propose a coordination model that exploits trust groups in order to promote safe interactions in the ubiquitous environment. Trust groups are asymmetric, that is, each device has its own view of the groups it belongs to, and long-lived, that is, their lifetime spans an extended period of time, despite group membership being dynamically handled. The dynamics of trust group creation, evolution and termination are described, based on the history of interactions of the device and on the ontology used to encode the context of trust. The programmer efforts required to reason about trust groups when coordinating mobile ad-hoc systems are discussed.