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. The first part is an introduction to aeronautical communications, their specific concepts, properties, requirements and terminology. The second part presents the currently used systems for air ground communications in continental and oceanic area. The third part enlightens the reader on the emerging and future communication systems and some leading research projects focused on this scope. Finally, before the conclusion, the fourth part gives several main challenges and research directions currently under investigation.