Dityrosine is a fluorescent molecule formed as a result of normal posttranslational processing. In many structural proteins, dityrosine confers resistance to proteolysis and physicochemical trauma as a stabilizing crosslink. Dityrosine has also been found in oxidative/nitrative stress under a variety of conditions and biological systems. In this regard, it has been used as an important biomarker for oxidatively modified proteins during UV and gamma-irradiation, aging, and exposure to oxygen free radicals, nitrogen dioxide, peroxynitrite, and lipid hydroperoxides. Renewed interest in dityrosine and other tyrosine oxidation products as clinical indicators of oxidative modification has driven the development of important techniques for the specific analysis and quantification of these molecules. The presence of elevated levels of dityrosine in mammalian tissue and urine samples has been measured by chromatographic separation followed by mass spectrometry GC-MS and HPLC-MS/MS. Increases in dityrosine levels have been associated with pathologies such as eye cataracts, atherosclerosis, acute inflammation, and Alzheimer's disease. The continued development of, and increased accessibility to, improved mass spectrometric instrumentation will expand the capability, feasibility, and sensitivity with which specific biomarkers like dityrosine can be measured.