Flavonoids are phytochemicals possessing antioxidative properties, but without toxicity in humans and animals. They are abundant in fruits and vegetables. This article summarizes properties of flavonoids concerning their distribution in plants, catabolism, and the structure-activity relationships. Flavonoids act as antioxidants via their free radical scavenging activity. They also inhibit lipid peroxidation, lower blood cholesterol level, and decreased atherosclerosis and cardiovascular diseases. Flavonoids are anticarcinogenic, while some flavonoids have estrogen-like activities. Uptake of dietary antioxidative agents to decrease the risk of cancer, cardiovascular, and other diseases plays an important role in preventive medicine.