Flavonoids are a large group of ubiquitous molecules synthesized by plants. Recent research has revealed that these molecules possess antioxidant activities which prevent free-radical damage to biological molecules like lipids, proteins and DNA, damage which can cause many cardiovascular and neurodegenerative diseases as well as cancer and diabetes. The underlying mechanisms involve metal chelation, free-radical scavenger capacities and inhibition of freeradical producing enzymes. Structure-activity studies have demonstrated common features of flavonoids that are highly important to their different activities. They include their planar structure; the number and position of their hydroxyl groups as well as the presence of the C2-C3 double bond. Many research groups indicated that flavonoids have shown cardioprotective activity, mainly through inhibition of lipid peroxidation. More recently, flavonoids are proven to be effective as anti-cancer agents, through apoptosis by induction of cell cycle arrest and inhibition of key enzymes involved in tumor promotion. Although discrepancies between in vitro and in vivo studies have been found, the elucidation of the mechanisms involved in flavonoid action will have many health benefits to man.