8-Hydroxy-2-deoxyguanosine (8-OHdG), generated as a result of reactive oxygen species (ROS)-mediated DNA damage repair, is the most widely recognized biomarker of oxidative damage to DNA, since guanosine is the most oxidized nucleobase in DNA. As a crucial marker of oxidative DNA damage, 8-OHdG has potential for assessing severity and prognosis in diseases and has been associated with a variety of pathologies, including chronic kidney disease, heart failure, and various cancers. This chapter focuses on the measurement of 8-OHdG concentrations as a key biomarker of oxidative stress. Although high-performance liquid chromatography (HPLC) has conventionally been used for the measurement of 8-OHdG concentrations, enzyme-linked immunosorbent assays (ELISA) have become increasingly widespread. This chapter presents a detailed protocol for the measurement of 8-OHdG concentrations using a highly sensitive ELISA kit suitable for serum/plasma and urine.