Pilots have to read and analyze much flight information. Conditions that require instant action can be instantly recognized by the use of indicator lights. These lights are grouped into annunciator systems, which have changed from simple, single indicator warnings to complex, multichannel systems. These systems provide an analysis of significant conditions of the aircraft and warn of any changes in this condition. This paper discusses the categories of annunciator indicators, circuit requirements, circuit design, additional input circuits, and application.