Modification of low density lipoproteins (LDL) by oxidation of their polyunsaturated lipid components has been implicated in the etiology of atherosclerosis. Oxidation proceeds by interaction of polyunsaturated fatty acids (PUFA) with active oxygen species or with lipid oxidation products. Moreover, there are indications that lipid oxidation promotes pre-mature coronary atherosclerosis since susceptibility to LDL oxidation has been associated with the severity of coronary heart disease. Protection against oxidation can come from antioxidants, antioxidant enzymes and co-factors. The use of antioxidants to inhibit LDL oxidation has been reviewed extensively. Under in vivo conditions, there exists a competition between oxidative and protective processes that depend on PUFA composition and on antioxidant levels. Therefore, it is important to properly assess the oxidation of the fatty acids in marine oils.