Endogenous and exogenous reactive species react with DNA in living organisms by numerous mechanisms and cause oxidatively induced DNA damage with multiple lesions. If not repaired, DNA lesions can cause genetic instability, leading to mutagenesis and cell death, and consequently to disease processes including carcinogenesis. Living organisms have evolved to repair DNA damage by numerous repair mechanisms, the activities of which depend on the nature of DNA lesions. The basic steps of DNA repair consist of lesion recognition and excision, followed by DNA synthesis and ligation. DNA repair pathways are drug targets in cancer therapy for enzyme inhibition to prevent drug resistance and to achieve optimal treatment. An overview is presented on the mechanisms of oxidatively induced DNA damage with resulting DNA lesions and on the repair pathways that process this type of damage to restore DNA structure.