IN TRADITIONAL COMPUTING ENVIRONMENTS, USERS actively choose to interact with computers. Ubiquitous computing applications are likely to be different-they will be embedded in the users' physical environments and integrate seamlessly with their everyday tasks. This vision leads to a set of defining characteristics, requirements, and research challenges for ubiquitous applications. This article identifies some of the key characteristics via a possible real-world scenario and derives the important application design and software infrastructure challenges that must be addressed by the computing research community.