This review examines power quality improvement in renewable energy systems using Flexible AC Transmission Systems (FACTS) devices. With the increasing integration of renewable sources like wind and solar into the power grid, maintaining high power quality is essential. FACTS devices play a crucial role in mitigating issues such as voltage fluctuations, harmonics, and reactive power imbalance. This paper explores various FACTS technologies, their applications, and their effectiveness in enhancing power quality in renewable energy systems. The review highlights recent advancements, potential challenges, and future research directions in optimizing power quality through FACTS.