In humans, approximately 10(7) cells divide per second. Estimates suggest that spontaneous mutations arise in about a third of those cells. These mutations arise as mistakes in DNA replication and when DNA polymerases copy damaged templates. The latter result from chemical hydrolysis of nucleoside bases or by reaction of DNA with electrophiles or reactive free radicals generated during metabolism (endogenous DNA damaging agents). This article highlights recent discoveries and emerging opportunities in the study of endogenous DNA damage and mutation.