This chapter discusses that virtual reality (VR) is reliant on many technologies, such as: (1) real-time 3D computer graphics, (2) wide-angle stereoscopic displays, (3) viewer (head) tracking, (4) hand and gesture tracking, (5) binaural sound, (6) haptic feedback, and (7) voice input/output. Of these, the first three are mandatory, the fourth is conventionally used but under some circumstances may not be necessary, and items five and six are becoming increasingly important to researchers in the field. Voice input/output technology is of immense utility but not a key technology that characterizes VR. The chapter describes the way such a system differs from that of conventional computer systems. A VR system is not just a bigger, faster, or better version of a conventional computer graphics (CG) system. In conventional CG systems, the user is like an external observer, looking in through a window at some synthetic environment. The synthetic environment that the VR user enters is defined with respect to a global zero-point and has built in conventions for directions. With head tracking, the VR user's kinaesthetic senses and visual system can be re-coupled. As the VR user turns his head, the viewing frustum and, hence, the display is updated. An added advantage of head tracking is that in addition to binocular parallax, one gets motion parallax.