Vitamin B3 (nicotinamide and nicotinic acid) is essential to all living cells. Vitamin B3 is biosynthetically converted to nicotinamide adenine dinucleotide (Fig. 1, NAD+), a versatile acceptor of hydride equivalents to form the reduced dinucleotide, NADH. The phosphorylated forms of the nicotinamide dinucleotides (NADP/NADPH) perform similar chemical functions within cells, although these are generally used in biosynthetic pathways and in cell protection mechanisms against reactive oxygen species. NAD(P)H provides reducing equivalents for cellular biochemistry and energy metabolism. Within eukaryotic cells, energy metabolism is largely mediated by electron transport chains found within the mitochondrion, and NADH plays a vital role in furnishing reducing equivalents to fuel oxidative phosphorylation. Thus, cellular energy metabolism is substantially mediated by vitamin B3-derived cofactors, and a large fraction of anabolic and catabolic pathways incorporates these cofactors within them.