Citrus by-products, such as peels, seeds, and pulp, are rich in bioactive compounds, including phenolics, flavonoids, and ascorbic acid. These naturally occurring antioxidants play a crucial role in reducing oxidative stress and promoting overall health, making them valuable for applications in the food and nutraceutical industries. To accurately assess their antioxidant potential, various in vitro assays, such as DPPH, FRAP, ABTS, TEAC, and ORAC, are employed, each relying on distinct reaction mechanisms. This chapter explores these analytical methods, outlining their underlying principles, benefits, and limitations to assist researchers in selecting the most suitable approach. A deeper understanding of these techniques is essential for enhancing the utilization of citrus by-products, supporting waste reduction efforts, and advancing sustainable innovations in food and pharmaceutical applications.