In many popular visions of ubiquitous computing, the environment proactively responds to individuals who inhabit the space. For example, a display magically presents a personalized advertisement, the most relevant video feed, or the desired page from a secret government document. Such capability requires more than an abundance of networked displays, devices, and sensors; it relies implicitly on recommendation systems that either directly serve the end user or provide critical services to some other application. This article appears in the October 2003 issue of IEEE Computer magazine. The author is David McDonald of the University of Washington. Ubiquitous Recommendation Systems | IEEE Computer, October 2003 | 
