Much of the past pervasive computing research has been devoted to systems composed of a small number of devices that interact with a single individual or a small group of users. Body-worn sensors and sensors embedded in the user's environment were used to infer user location, activity, and information about the user's immediate surroundings, shaping the concept of context awareness. This issue is motivated by the fact that as technology becomes truly pervasive, we must proceed from the "single user, single system" perspective to "large-scale heterogeneous systems" involving many devices and many individuals collaborating over different spatial and temporal scales.