Quercetin is a flavonol, a subclass of flavonoids, widely distributed in plants including onions, apples, berries, and tea. It exhibits antioxidant, anti-inflammatory, and potential cardioprotective properties by scavenging free radicals and modulating signaling pathways. Chemically, it features a 3-hydroxyflavone backbone with five hydroxyl groups enabling hydrogen bonding and metal chelation. It is poorly absorbed in the intestine but metabolized to conjugates. Kaempferol is a closely related analog differing by one hydroxyl group. Both contribute to the health benefits of plant-based diets.