Sirtuins, possessing either histone deacetylase or mono-ribosyltransferase activity, regulate important pathways in bacteria, archaea and eukaryotes. SIRT6, an enzyme highly expressed in skeletal muscles, brain, heart, liver, and thymus, affects transcriptional regulation in a tissue-specific manner. This enzyme has a two-domain structure that consists of a large Rossmann fold and a smaller and structurally more varied sequence containing a Zn(2+)-binding motif. The C-terminus is required for proper nuclear localization, while the N-terminus is important for chromatin association and for intrinsic catalytic activity. SIRT6 promotes resistance to DNA damage and oxidative stress, the principal defects associated with age-related diseases. The modulation of aging and other metabolic functions by SIRT6 may be indicative of previously unrecognized regulatory systems in the cell. The propensity of individual SIRT6 molecules to undergo intramolecular mono-ADP-ribosylation, suggests this auto-ribosylation may contribute to the self-regulation of SIRT6 function. Until recently, SIRT6 was an orphan enzyme whose catalytic activity and substrates were unclear. It was known that, similar to the yeast Sir2 protein, human SIRT6 deacetylates histones and regulates DNA stability and repair; however, new mechanistic insights can be derived from the discovery of the highly substrate-specific histone deacetylase activity of SIRT6. This deacetylase activity promotes proper chromatin function in several physiologic contexts, to include telomere and genome stabilization, gene expression and DNA repair. By maintaining both the integrity and the expression of the mammalian genome, SIRT6 may help prevent cellular senescence. Moreover, successful molecular modulation of SIRT6 activity may lead to the development of new chemotherapeutic modalities. The action of SIRT6 is described in this review, with an emphasis on the cellular roles of the enzyme and the relation of those enzymatic functions to human biology and disease.