The growing diffusion of portable devices with wireless connectivity and the integration of telecommunication systems and the Internet enable users to benefit from anytime and anywhere impromptu collaboration. Security is crucial to ensure secure sharing of information, but calls for novel solutions capable of establishing trust relationships on-the-fly among previously unknown entities. Traditional trust solutions seem to be inappropriate in dynamic mobile environments. They typically rely on centralised trust certification authorities and assign to entities levels of trust that depend on static entity attributes, such as entity identity or role. However, entity identities/roles are often either not sufficiently informative or too limited to take adequate collaborative decisions. This paper presents a novel trust model that rules the assignment of trust to entities based on various dynamic context information, e.g., user/device location, device properties, user needs, local operating conditions. As key features, this model allows to determine, to associate and to adapt entity levels of trust depending on the contexts where users operate and users acquire/lose a level of trust when entering/leaving a specific context. The proposed model facilitates the update of trust relationships as relevant changes in context information occur. The paper also shows the implementation of the proposed model in the COMITY framework that provides support facilities for defining and enforcing context-based trust policies.