There is now rather convincing evidence that oxygen free radicals are involved in some aspects of aging, arthritis, and tumor initiation and promotion. Oxygen free radicals may also be involved in tissue damage occurring in ischemia, ionizing irradiation, ozone damage, high-pressure oxygen therapy, and processes involved in leukocyte phagocytosis as well as in the toxic action of several drugs. It is important to examine oxygen free radical reactions with DNA as primary mediating aspects of the action of these species. One product formed by hydroxyl free radical addition to DNA, (8-OHdG), can be very sensitively quantitated using LCED methodology. The presence of this modified nucleoside within DNA may have important biological consequences. This chapter is concerned with aspects of LCED methodology as a technique to quantitate the 8-OHdG content in the DNA of biological systems.