The book is aimed at maintenance technicians working with ILS as well as planners of navigation aids and for flight inspection crew. It is a necessary prerequisite for further equipment course or training course in computer ILS simulations. After studying this book the reader will be able to understand the basic ILS principles for localiser and glide path systems, understand relevant ICAO specification and be able to do basic fault finding and call for expert assistance in a qualified way. The reader will understand the functions and limitations of the airborne equipment as well as flight inspection reports and the meaning of the reported values. The most important features of the localiser and glide path will be discussed, and the reader will be soon be ready to receive the On-the-Job and equipment specific training required to perform independent maintenance work on ILS systems.