Non-orthogonal multiple access (NOMA) has received significant attention due to its capability to enhance spectral efficiency and support diverse services with distinct quality-of-service requirements. This book chapter aims to provide a comprehensive overview of NOMA, focusing on its principles, advantages, and applications in the context of 5G networks and future communication systems. It delves into the fundamental concepts underlying NOMA, such as power domain multiplexing and successive interference cancellation (SIC), elucidating its benefits in terms of increased connectivity, enhanced throughput, and better user experience. Further, code domain techniques such as sparse code multiple access (SCMA) and pattern division multiple access (PDMA) are also introduced. Moreover, this chapter discusses the challenges and open research issues associated with deploying NOMA in practical scenarios. The exploration of NOMA’s potential enhancements and its role in the evolution toward 6G networks will be elucidated, offering insights into the future trajectory of wireless communication systems.