Determining the flavonoid content in citrus by-products is essential for evaluating their potential health benefits and optimizing their use in nutraceuticals, functional foods, and pharmaceuticals. Citrus by-products represent a valuable and underexploited source of flavonoids, which can be extracted using various methods such as solvent extraction followed by quantification by colorimetric, HPLC, or spectrophotometric assay. The development of efficient and sustainable methods to extract and quantify flavonoids from citrus by-products not only offers opportunities for the valorization of food waste but also contributes to the growing field of natural antioxidants and nutraceuticals.