Recent technological advances have made it possible to have large wireless ad-hoc networks of small devices. The prime purpose of such sensor networks is to disseminate information about the environment they sense. We observe that information provided may have different levels of importance and argue that the sensor networks should be willing to spend more resources in disseminating packets carrying more important information. In this paper, we introduce the concept of service differentiation as applicable to sensor networks, highlight its fundamental differences with normal data networks and define the relevant metrics. Various mechanisms which may provide services compatible with the packet priority levels are proposed. We evaluate one of the proposed method, Adaptive Forwarding Scheme (AFS) and show that our scheme provides the desired levels of service.