The emerging reality of wireless sensor networks deployed as long-lived infrastructure required to serve multiple applications necessitates the development of fine-grained security support. Specifically, to allow sensor nodes to participate in multiple concurrent applications, access control is required on a per-application basis. This paper presents a policydriven security architecture for wireless sensor networks that addresses the concern of fine-grained access control and secure deployment of security policies, while respecting the resource-constrained nature of wireless sensor networks. A prototype of this system has been realized and evaluated using the LooCI component model and the Sun SPOT sensor network platform. Categories and Subject Descriptors D.4.6 [Security and Protection]: Access controls, Authentication General Terms Security, Design, Performance Keywords Security, Component models, Policy-based Management