This book explores different facets of millimeter wave systems, which form a central part of 5G systems. It explains how these systems serve as a foundational building block of 5G-Advanced/6G as these systems evolve. Millimeter Wave Communications in 5G and Towards 6G focuses on millimeter wave channel modeling, radio frequency (RF) and antenna level constraints imposed on beamforming, beamforming design for link level incorporating the RF/antenna constraints and the channel structure, as well as system level deployment considerations. With significant academic and industrial experience, the authors are well-equipped in explaining how the millimeter wave research developed, the fundamental principles/math beneath the technology, and in explaining precisely the "Why?" behind the "What?" that make the 5G-NR specifications. The authors examine point-to-point systems at a single link level and show how the traditional sub-6 GHz-based beamforming procedures simplify to a simplistic signal processing approach of directional beam scanning. This book examines the foundational background that led to specific choices in the millimeter wave part of the 5G-NR spec as well as chart out the roadmap in terms of future research and development activities in this arena. The book ends by providing a scope of future research in this area. This book is geared towards both introductory as well as advanced researchers in both industry and academia working in the areas of 5G, 5G-Advanced and 6G communications. It would also be useful for senior undergraduate and graduate students in universities focusing on wireless communications topics.