Sirtuins are NAD(+)-dependent protein deacetylases that are involved in transcriptional regulation, metabolism, apoptosis, differentiation and ageing. These unique enzymes are inhibited by nicotinamide, which is a form of vitamin B-3. Recent studies have uncovered the molecular basis for nicotinamide inhibition, and provided the framework to understand the physiological processes mediated by sirtuins and to develop strategies to modulate their cellular activity.