Sirtuin 1 (SIRT1) is an NAD+-dependent histone deacetylase which regulates many normal physiological and pathological processes. In addition to its place in cellular energy metabolism, increasing evidence shows a role for SIRT1 in tumorigenesis, wherein it can function as either a tumor promoter or suppressor. Its function in malignancy varies with concentration, cellular location, temporal and spatial distribution, and regulation by upstream and downstream factors. In this mini-review, we present the existing data which implicates SIRT1 in tumorigenesis.