User interface (UI) design is a critical component of any virtual environment (VE) application, and especially for VE applied to medicine. User interfaces for VE are becoming more diverse. Mice, keyboards, windows, menus, and icons--the standard parts of traditional WIMP interfaces--are still prevalent, but nontraditional devices and interface components are proliferating rapidly. These include spatial input devices such as trackers, 3-D pointing devices, and whole-hand devices allowing gestural input. Three-dimensional, multisensory output technologies--such as stereoscopic projection displays, head-mounted displays (HMDs), spatial audio systems, and haptic devices--are also becoming more common. In this chapter we present a brief overview of 3-D interaction and user interfaces technologies for VE.