Radio frequencies have become a basic resource for the development of the information society. In fact, radio waves are a mandatory vehicle in order to carry the message to customers and a truly worldwide communication needs their properties. Given the market demands for more and more frequencies, means have to be found to share this limited resource most effectively and to continuously improve its efficiency. Radio spectrum management is thus a major objective for our modern world. This book describes the current tools for spectrum management with their fundamental technical and legal basis. It outlines the global evolution of radio services in their different application domains and introduces the actors who contribute to the collective management of the spectrum. It also discusses the main questions these actors have to deal with and answer in order to design for the future.