Publisher Summary This chapter describes the antioxidant activity by headspace gas chromatography (GC) of volatile oxidation products of ω-6 and ω-3 polyunsaturated lipids. The technique of headspace GC is rapid and suitable for biological materials because no sample workup is necessary. This method can be used to determine the specific oxidation of ω-3 and ω-6 polyunsaturated fatty acids (PUFA). It is ideal for the measurement of oxidative susceptibility as affected by diet and etiology of biological samples. This method may prove to be valuable for routine checking of antioxidant supplementation and clinical etiology. A wide range of phenolic compounds have been shown to inhibit the in vitro oxidation of human low-density lipoproteins (LDL), which plays an important role in the initiation of atherosclerosis. Plant phenolic antioxidants in fruits and vegetables may also have a protective effect on coronary heart disease and cancer, but the molecular basis of protection is not understood. A better knowledge of the mechanism of natural antioxidants requires more systematic studies with specific methods providing specific chemical information that can be related directly to oxidative modifications of biological systems. Pharmacokinetic data are needed to evaluate the effectiveness of the polyphenolic antioxidant compounds in fruits and beverages and their potential role in reducing coronary heart disease.