This book provides a comprehensive exploration of Non-Orthogonal Multiple Access (NOMA) in Heterogeneous Cellular Networks (HCNs), focusing on both theoretical foundations and practical considerations. It examines critical challenges such as non-linear high-power amplifiers (HPAs), and imperfections in successive interference cancellation (SIC). Furthermore, it presents advanced techniques including simultaneous wireless information and power transfer (SWIPT) with cooperative NOMA and the application of reconfigurable intelligent surfaces (RIS) for enhancing coverage, reliability, and energy efficiency.