The proposed book will provide a comprehensive picture of the journey from 5G networks towards 6G. Different aspects, ranging from theoretical foundations to existing platforms and technologies will be presented and analysed, by critically highlighting the real opportunities and the challenges. The first chapters are to describe the general "transitioning" aspects from 5G to 6G. After that, an overview on the different technologies is provided, to present at the end the security aspects, the new applications and an analysis of the electromagnetic exposure, above all in terms of comparative in respect of 5G. Security and Application will be straightforward better explained after the big picture on the different technologies is detailed in the previous chapters. 6G is an emerging complex paradigm, with some important new concepts, that would enable an extremely high reliability and low latency. The main purpose is to provide the reader a thorough vision of the enabling technologies, their potential and what is still needed to fill the gap in terms of sustainability and coexistence of the different technologies. Since different emerging technologies will be considered and detailed in the book, it will be interesting for the reader to infer the major opportunities of each and the main limitations. This kind of information will mainly boost the potential interactions of different technologies to provide a more reliable system.