Oxidative damage DNA is aninevitable, natural consequence of cellular metabolism resulting from formation of reactive oxygen species (ROS) including free oxygen radicals. However, the level ofthe damage may increase under conditions of oxidative stress, arising from exposure to a variety of physical or chemical insults. In this review we present the mechanisms by which oxidative damage to DNA may lead to pathological processes involved in the development of cancer, cardiovascular diseases and ageing. Furthermore, we describe mechanisms of DNA repair which play a key role in maintaining cellular function upon DNA insult. Among over 20 identified and described oxidative modifications of DNA bases only one derivative, namely 8-oxo-2'-deoxyguanosine (8-oxo-dG), has become a subject of intense research. Therefore, we are presenting methods of 8-oxo-dG detection as a marker of oxidatively damaged DNA.