As a general rule of thumb, every 10 years the cellular industry introduces a new technology: 3G Universal Mobile Telecommunications Service circa 2000, 4G Long-Term Evolution circa 2010, and now finally 5G in 2020. The Radio Access Network (RAN) is arguably the most important component in a mobile network. When it comes to deployments, RAN is what eventually makes it possible and economically feasible (or not). This introduction presents an overview of the key concepts discussed in this book. The book describes the technological, regulatory, and business driving forces behind 5G in general and how these diverse requirements, challenges, and marketing considerations affect the RAN. It provides a high-level overview of all the components of a 5G system: the core network, the air interface protocol stack, and the air interface physical layer. The book is dedicated to various technologies that are not always considered part of RAN architecture but are nevertheless fundamental to RAN deployments.