The oxidative modification of proteins by reactive oxygen species (ROS) and other reactive compounds is associated with a number of disease and pathophysiological processes as well as aging (1). Under physiological conditions, almost all oxidative modifications of proteins are resulting in an increase of carbonylated proteins. The three major pathways leading to carbonyl group formation (protein oxidation) are shown in Fig. 1. Carbonyl groups are introduced into proteins as a result of: 1) metal catalyzed oxidation of amino acid residues; 2) lipid peroxidation (the Michael addition of protein amino, sulfhydryl, and imidazole groups to the double bond of α,β unsaturated aldehydes, which are produced during the oxidation of polyunsaturated fatty acids); and 3) protein glycation and glycoxidation reactions. The carbonyl content of proteins is therefore an index of the amount of oxidative protein damage attributable to either direct attack of free radicals or the modification of proteins by oxidation products of carbohydrates or polyunsaturated fatty acids (PUFAs).