Every decade seems to bring a new wave of technology; this was the case with the arrival of 3G and 4G. From this basic perspective, the arrival of 5G strengthens this “rule.” Perhaps one of the most important lessons learned from LTE is the need for a more flexible and scalable system toward the future. This is an intrinsic characteristic of 5G new radio (NR) design. This introductory chapter of the book covers the most important new development over LTE, the multi-antenna perspective. It presents the system requirements and a brief note on the 5G spectrum, explaining that in the first stage of 5G discussions, a broad set of technology components and design guidelines were considered. These included aspects of waveform, multiple access, multiple numerologies, and multi-antenna.