Proanthocyanidins, which belong to a class of polyphenols, are widely distributed throughout the plant kingdom. Most people ingest trace amounts of proanthocyanidins through foods such as red wine and cranberry juice. However, the functional properties of proanthocyanidins have been little understood. Since 1983, we have studied the antioxidative functions, preventive actions on diseases and utilization of proanthocyanidins. The antioxidative activities of proanthocyanidins were found to be much stronger than vitamin C or vitamin E in aqueous systems. The mechanisms for their antioxidative actions were shown to involve radical scavenging, quenching, and enzyme-inhibiting actions. The preventive actions of proanthcyanidins on diseases relating to reactive oxygen species was examined using animal tests. Proanthocyanidin-rich grape seed extract was showed to have preventive actions on diseases such as atherosclerosis, gastric ulcer, large bowel cancer, cataracts and diabetes. In human intervention trials, grape seed extract was shown to have preventive effects on the increase in lipid peroxides in human plasma after exercise and on muscle fatigue after training. The uses and manufacturing techniques of proanthocyanidin products were subsequently developed. The products were launched as antioxidants in food additives, ingredients in nutritional supplements, and cosmetics.