Flavonoids, a diverse class of polyphenolic compounds, are widely distributed in the plant kingdom and are renowned for their vast array of pharmacological properties, including antioxidant, anti-inflammatory, anticancer, and antimicrobial activities. This chapter provides a comprehensive exploration of the chemical structure, biosynthesis, and structural diversification of flavonoids, emphasizing the influence of enzymatic modifications and environmental factors on their complexity. The chapter delves into the mechanisms of structural diversification, including hydroxylation, glycosylation, methylation, and prenylation, which significantly affect their biological activity and solubility. Modern analytical and spectroscopic methods for flavonoid identification are discussed alongside advanced synthetic and semisynthetic strategies used to enhance their pharmacological potential. A dedicated section explores the integration of computational chemistry to understand structure–activity relationships and guide rational design of novel flavonoid derivatives. Furthermore, the chapter highlights emerging applications across pharmaceuticals, nutraceuticals, and cosmetics, while addressing key challenges such as poor bioavailability. By combining chemical, biological, and computational insights, this chapter offers a multidimensional perspective on the transformative potential of flavonoids in modern science.