Much of our insight has been gained from practical experience building ubicomp applications. Since the concrete details of particular systems will help in expressing challenges raised later on, we will describe here two projects. The first project, Classroom 2000, is an experiment to determine the impact of ubiquitous computing in education and involves the instrumentation of a room. The second project, Cyberguide, is a suite of flexible mobile tour guide applications that cover an area ranging in size from a building to a campus.