The maintenance of DNA integrity and genome stability is essential for the viability of cells and the life of organisms. The genetic material is constantly under attack from external environmental factors and endogenous metabolic products that can alter its chemical structure and nucleotides sequence. In this book, the authors present topical research in the study of DNA repair including topics such as DNA repair as the primary adaptive function of sex in bacteria and eukaryotes; DNA repair as a therapeutic target in breast cancer; DNA repair in the nervous system; targeting BRCA1 DNA repair pathway for cancer chemotherapy and DNA damage and repair during photodynamic therapy.