Oxidative stress is an inevitable process of the aerobic life. Among the effects of oxidative stress, protein oxidation takes an important place because of the high protein abundance in the organism. The degradation of non-functional, oxidized proteins is an essential function of intra- and extracellular proteolytic systems. However, severely oxidized proteins are poor substrates for degradation and may accumulate. This process of accumulation of oxidized, cross-linked protein material is involved in the physiology and pathophysiology of aging and many diseases.