This paper re-presents basic information regarding an instrument landing system (ILS), an ILS is a precision, radio navigation operates as a ground-based instrument approach system, using a combination of radio signals wherever as adopted by airports and airlines worldwide, which provides lateral and vertical guidance to an aircraft approaching and landing on a runway in weather conditions, that Otherwise might have cause in a missed approach and possible alternate to another airport, The aim of this paper is to describe and review of an instrument landing system (ILS), taken in consideration the previous studies on ILS, this paper will be as a foundation for the second paper dedicated for Evaluation of the Instrument landing system (Localizer system) used at Khartoum International Airport.