Since the discovery of 8-OH-dG formation, various aspects of oxidative DNA damage have been studied. In the present decade, we have studied the chemistry of oxidative DNA damage, its mutagenic effects, and its repair mechanisms. We are also analyzing 8-OH-dG levels in human samples, such as leukocytes and urine, as a marker of oxidative DNA damage, to evaluate the oxidative stress of individuals. In the meantime, many groups have initiated studies of the biological significance of 8-OH-dG, and more than 2000 papers about 8-OH-dG have been published thus far. In this review, we will summarize our resent research progress on oxidative damage.