The protection of wireless sensor networks is an important task, especially for critical situations such as intrusion detection, tamper monitoring, or military applications. To date, much of the emphasis on protecting sensor networks has focused on key management and secure routing. However, as sensors become more capable and ubiquitous, the need for more fine-grained control over their resources grows. In this paper, we present a security policy for wireless sensor networks which provide designers and administrators the ability to fine-tune access to sensor resources. We build on the notion of group-based key establishment to show how group membership can be utilized in deploying a dynamic, robust, and flexible security policy for wireless sensor networks.