The Wizard of Oz prototyping approach, widely used in human-computer interaction research, is particularly useful in exploring user interfaces for pervasive, ubiquitous, or mixed-reality systems that combine complex sensing and intelligent control logic. The vast design space for such nontraditional interfaces provides many possibilities for user interaction through one or more modalities and often requires challenging hardware and software implementations. The WOz method helps designers avoid getting locked into a particular design or working under an incorrect set of assumptions about user preferences, because it lets them explore and evaluate designs before investing the considerable development time needed to build a complete prototype.