With today’s and tomorrow’s wireless technologies, such as IEEE 802.11, BlueTooth, RF-Lite, and G3, mobile devices will frequently be in close, interactive communication. Many environments, including offices, meeting rooms, automobiles and classrooms, already contain many computers and computerized appliances, and the smart homes of the future will have ubiquitous embedded computation. When the user enters one of these environments carrying a mobile device, how will that device interact with the immediate environment? We are exploring, as part of the Pebbles research project, the many ways that mobile devices such as PalmOS Organizers or PocketPC / Windows CE devices, can serve as useful adjuncts to the “fixed” computers in the user’s vicinity. This brings up many interesting research questions, such as how to provide a user interface that spans multiple devices that are in use at the same time? How will users and systems decide which functions should be presented and in what manner on what device? How can the user’s mobile device be effectively used as a “Personal Universal Controller” to provide an easy-to-use and familiar interface to all of the complex appliances available to a user? How can communicating mobile devices enhance the effectiveness of meetings and classroom lectures? I will describe some preliminary observations on these issues, and discuss some of the systems that we have built to investigate them.