In this Book, we provide a comprehensive survey of the principle concept, the most recent development, and the future research directions of NOMA. Specifically, the basic principle and type of NOMA will be introduced at first. Then, the power-domain NOMA scheme is discussed in uplink and downlink, power allocation strategies and user-pairing schemes in NOMA. After that we discuss how NOMA performs when it is integrated with various wireless communications techniques, such as cooperative communications, multiple-input multiple output (MIMO).