Oxidative stress is apparent in pathology associated with aging and many age-related, chronic diseases, including atherosclerosis, diabetes mellitus, rheumatoid arthritis, and neurodegenerative diseases. Although it cannot be measured directly in biological systems, several biomarkers have been identified that provide a measure of oxidative damage to biomolecules. These include amino acid oxidation products (methionine sulfoxide, ortho-tyrosine (o-tyr) and dityrosine, chlorotyrosine and nitrotyrosine), as well as chemical modifications of protein following carbohydrate or lipid oxidation, such as N epsilon-(carboxymethyl)lysine and N epsilon-(carboxyethyl)lysine, and malondialdehyde and 4-hydroxynonenal adducts to amino acids. Other biomarkers include the amino acid cross-link pentosidine, the imidazolone adducts formed by reaction of 3-deoxyglucosone or methylglyoxal with arginine, and the imidazolium cross-links formed by the reaction of glyoxal and methylglyoxal with lysine residues in protein. These compounds have been measured in short-lived intracellular proteins, plasma proteins, long-lived extracellular proteins, and in urine, making them valuable tools for monitoring tissue-specific and systemic chemical and oxidative damage to proteins in biological systems. They are normally measured by sensitive high-performance liquid chromatography or gas chromatography-mass spectrometry methods, requiring both complex analytical instrumentation and derivatization procedures. However, sensitive immunohistochemical and ELISA assays are now available for many of these biomarkers. Immunochemical assays should facilitate studies on the role of oxidative stress in aging and chronic disease and simplify the evaluation of therapeutic approaches for limiting oxidative damage in tissues and treating pathologies associated with aging and disease. In this article we summarize recent data and conclusions based on immunohistochemical and ELISA assays, emphasizing the strengths and limitations of the techniques.