Virtual reality systems (often referred to simply as VR) are computer interfaces that provide users with the sensory experience of being in a simulated space. The goal is to provide a person with a compelling sense of being present in a computer-generated virtual world that is too expensive, too dangerous, or not practical to visit in person. A multiplicity of VR-based entertainment systems currently exist, ranging from video games to amusement park rides. VR is also playing an increasing role in applications such as training, mission planning, education, and rehabilitation, where it is important that users act in a virtual world in the same manner as they would if placed in the corresponding physical world that is being simulated. This entry describes visual displays, perception ...