Wireless connectivity and widespread diffusion of portable devices offer novel opportunities for users to share resources anywhere and anytime and to form ad-hoc coalitions. Resource access control is crucial to leverage ad-hoc collaboration. In pervasive scenarios, however, collaborating entities cannot be known in advance and resource availability frequently vary, even unpredictably, due to user/device mobility, thus complicating resource access control. Access control policies cannot be defined based on entity's identities/roles, as in traditional access control solutions, and cannot be all specified a priori to face any operative run time situation, but require continuous adjustments to adapt to the current. To address these issues, this paper advocates the adoption of novel access control policy models that follow two main design guidelines: context-awareness to control resource access on the basis of context visibility and to enable dynamic adaptation of policies depending on context changes, and semantic technologies for context/policy specification to allow high-level description and reasoning reason about context and policies. The paper describes also the implementation of these concepts in a semantic context-aware policy model that adopts ontologies and rules to express context and context-aware access control policies and to allow policy adaptation.