Vegetable oils are chemically unstable due to the sensitivity to oxidation of their unsaturated fatty acids. The oxidative mechanisms are based on complex and radical reactions that always result in a significant loss of oil quality in both sensorial (rancidity) and nutritional values (loss of polyunsaturated fatty acids and vitamin E). Monitoring the oxidation state of oil can be performed using different markers, each bringing a partial information of the whole phenomenon, but unable to predict by itself the future stability of the oil. The accelerated ageing tests used in reasonable heating conditions are useful for measuring the resistance of oil to oxidation but are difficult to extrapolate to normal storage conditions. The oxidative degradation of oil during storage can be delayed by protecting it from light, heat, oxygen and metals. Another way consists to increase the retention of endogenous antioxidants naturally contained in the oil seeds, such as tocopherols (vitamin E), which are partially removed during conventional operations of extraction and refining.