This book deals with air-ground aeronautical communications. The main goal is to give the reader a survey of the currently deployed, emerging and future communications systems dedicated to digital data communications between the aircraft and the ground, namely the data link. Those communication systems show specific properties relatively to those commonly used for terrestrial communications. In this book, the system architectures are more specifically considered from the access to the application layers as radio and physical functionalities have already been addressed in detail in others books.