Flavonoids are a large and varied group of natural products, with more than 10,000 identified compounds (Hao et al. 2024; Prabhu et al. 2025). As plant secondary metabolites, they are ubiquitous throughout the plant kingdom, found in fruits, vegetables, grains, bark, roots, flowers, and beverages such as tea and wine (Diwan et al. 2017; Seo et al. 2014). Chemically, they are characterized by a polyphenolic C6–C3–C6 backbone, consisting of two aromatic rings (A and B) interconnected by a three-carbon heterocyclic ring (C) (Khan et al. 2025). The structural variations within this framework, primarily the oxidation state of the C ring and the substitution patterns on all three rings, give rise to the major subclasses that dictate their biological properties. These subclasses include flavanols (e.g., quercetin), flavones (e.g., luteolin), flavan-3-ols (e.g., catechins), flavanones (e.g., hesperetin), anthocyanidins, and isoflavones (e.g., genistein) (Fig. 17.2) (Diwan et al. 2017; Dongiovanni et al. 2016). In their natural context, flavonoids serve a multitude of critical functions for the host plant. They are responsible for the vibrant pigmentation of flowers and fruits, which serves to attract pollinators and seed dispersers (Harborne and Grayer 2017). Furthermore, they act as crucial defense molecules, protecting plants from a wide array of biotic and abiotic stressors (Shah and Smith 2020). They function as potent ultraviolet filters, phytoalexins to combat microbial pathogens, and signaling molecules in plant–microbe interactions (Das et al. 2024). This inherent bioactivity, honed by evolutionary pressures, forms the basis of their profound pharmacological potential in human health.