Many Ubiquitous Computing applications require tags and sensors that track the daily activates of people and things. For example, tags might be placed on objects to allow a system to remind people of the object’s location. In another application, caretakers of the elderly may wish to monitor the daily activities of those under their care, in order to track decline in functioning and offer appropriate medical care. These forms of tracking, however, raise privacy concerns. A person does not want to be tracked by everybody, but rather only by those that are trusted and only during certain times. We present a method of building tags and tracking devices that ensure a person is tracked only by those that have been granted that right, and only during the times specified.