Abstract: Various endogenous and exogenous sources can induce damage to DNA, which has been implicated in the pathogenesis of many human diseases including cancer and ageing. In this presentation, I will discuss the use of mass spectrometry, together with selective stable-isotope labeling, for the elucidation of the structures of novel DNA damage products. In addition, the use of LC-MS/MS with the stable isotope dilution technique for the quantification of DNA lesions formed in vitro and in vivo will be presented. Moreover, we will discuss our recent applications of LC-MS/MS, coupled with shuttle vector technology, for assessing the replication and repair of DNA lesions in bacteria and human cells. Through this presentation, we hope to demonstrate that mass spectrometry has become an indispensable tool for studying DNA damage and mutagenesis.