Sir2-like enzymes, also known as sirtuins, comprise a universally conserved family of NAD⁺-dependent protein deacetylases that play important roles in transcriptional silencing, fat mobilization, metabolic regulation, and lifespan extension. NAD⁺ is cleaved during the deacetylation reaction, yielding nicotinamide, deacetylated peptide and O-acetyl ADP-ribose products. In some cases, sirtuins catalyze mono-ADP ribosylation of their substrates rather than deacetylation. In an effort to determine the enzymatic mechanism of the NAD⁺-dependent deacetylation reaction, we have determined structures of sirtuins bound to a variety of substrates and products, as well as to a transition state analogue. In addition, we have trapped a key covalent reaction intermediate in complex with the enzyme and solved its structure. These structures provide insights into the structure-based mechanism of NAD⁺ cleavage and deacetylation. These mechanistic insights have been extended to explain the second reaction, ADP-ribosylation, that is catalyzed by some sirtuins. In solution studies of a trypanosomal sirtuin, we have identified the side chains that are ADP-ribosylated by TbSir2 and present a plausible mechanism for the dual activity based on our structural studies.