Nicotinamide adenine dinucleotide (NAD+, also nadide or beta-NAD) is a coenzyme found in all living cells. It carries electrons in reduction-oxidation reactions and is a co-substrate for non-redox enzymes, including the sirtuins, poly(ADP-ribose) polymerases (PARPs) and CD38. Reviews describe a decline in tissue and cellular NAD+ levels with age in several model organisms and summarise research on how NAD+ synthesis and consumption are regulated. Studies in rodents have examined raising NAD+ levels in ageing mice and the effects of NAD+ itself in brain ischaemia models.