As computing devices and their users become increasingly mobile, the demand for information about the application's environment, or context, becomes significantly important to the efficient and robust operation of mobile and pervasive computing systems. Applications must be able to adapt themselves to changing conditions to satisfy users' demands and expectations and to ensure that the application's resource usage matches the environment's capabilities. Sensing context using traditional means incurs network communication, which competes with the applications using the network and expends valuable network resources, especially communication bandwidth and battery power. In this project, we are exploring passively sensing context metrics. This results in measurements that are basically approximations of actual context, but can be collected with zero cost in terms of network communication. This project develops a model of passive context sensing and a general framework for building and deploying passively sensed context metrics.