The fifth generation of mobile communication became a reality during 2019 as the 3rd Generation Partnership Project released the first set of Release 15 Technical Specifications and respective equipment, both network elements and mobile devices, to be available for commercial deployments. This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book presents new key functionalities of Release 16 that complement the first phase of 5G. It is thus an addition to the contents of the already published 5G Explained book, providing further descriptions to understand the complete picture of the full version of 5G. The book presents up-to-date functionalities of Release 16, and some of the indications of the technological topics under development for the forthcoming Release 17 and beyond.