This paper discusses the potential and challenges of Power-Domain Non-Orthogonal Multiple Access (NOMA) in 5G systems. NOMA is a promising technique for enhancing performance in next-generation cellular communications, offering greater spectrum efficiency compared to Orthogonal Frequency Division Multiple Access (OFDMA). The paper focuses on power-domain NOMA, which uses superposition coding (SC) at the transmitter and successive interference cancellation (SIC) at the receiver. It reviews recent progress in NOMA for 5G, including capacity analysis, power allocation strategies, user fairness, and user-pairing schemes. The paper also discusses how NOMA performs when integrated with other wireless communication techniques such as cooperative communications, MIMO, beamforming, space-time coding, and network coding. It addresses several important issues in NOMA implementation and provides avenues for future research. The paper also covers various aspects of NOMA, including its impact on path loss, cooperative NOMA, fairness in NOMA, NOMA with MIMO and beamforming, NOMA in a coordinated system, network NOMA, NOMA user pairing, and energy-efficient NOMA. The paper concludes that NOMA is a promising multiple-access technique for future radio access due to its ability to achieve higher system-level throughput and user fairness.This paper discusses the potential and challenges of Power-Domain Non-Orthogonal Multiple Access (NOMA) in 5G systems. NOMA is a promising technique for enhancing performance in next-generation cellular communications, offering greater spectrum efficiency compared to Orthogonal Frequency Division Multiple Access (OFDMA). The paper focuses on power-domain NOMA, which uses superposition coding (SC) at the transmitter and successive interference cancellation (SIC) at the receiver. It reviews recent progress in NOMA for 5G, including capacity analysis, power allocation strategies, user fairness, and user-pairing schemes. The paper also discusses how NOMA performs when integrated with other wireless communication techniques such as cooperative communications, MIMO, beamforming, space-time coding, and network coding. It addresses several important issues in NOMA implementation and provides avenues for future research. The paper also covers various aspects of NOMA, including its impact on path loss, cooperative NOMA, fairness in NOMA, NOMA with MIMO and beamforming, NOMA in a coordinated system, network NOMA, NOMA user pairing, and energy-efficient NOMA. The paper concludes that NOMA is a promising multiple-access technique for future radio access due to its ability to achieve higher system-level throughput and user fairness.