During the 1970s, studies were carried out by accident investigators and regulatory authorities to examine one of the most significant causes of aircraft accidents of the time: controlled flight into terrain (CFIT). This can be defined as an accident where a serviceable aircraft, under the control of a qualified pilot, inadvertently flies into terrain, an obstacle or water. CFIT accidents usually occur during poor visual conditions, often influenced by other factors, e.g. flight crew distraction, malfunctioning equipment or air traffic control (ATC) miscommunication. With CFIT, the pilots are generally unaware of this situation until it is too late. The outcome of these investigations was that many CFIT accidents could be avoided with a ground proximity warning system (GPWS). A system was developed in 1967 to alert pilots that their aircraft was in immediate danger of CFIT. This system was further developed into the enhanced ground proximity warning system (EGPWS) by adding a forwardlooking terrain avoidance (FLTA) feature, made possible via global positioning system technology. This chapter describes the generic name given to this type of protection: terrain awareness warning system (TAWS).