β-Nicotinamide mononucleotide (β-NM) is an important intermediate metabolite in the nicotinate and nicotinamide metabolism pathway. Mammals predominantly use nicotinamide rather than nicotinic acid as a precursor for NAD biosynthesis. Instead of the deamidation to nicotinic acid, nicotinamide is directly converted to β-Nicotinamide mononucleotide by nicotinamide phosphoribosyltransferase (NAMPT, EC 2.4.2.12). The enzyme nicotinamide mononucleotide adenylyltransferase (NMNAT, EC 2.7.7.1), which is a member of the nucleotidyltransferase alpha/beta-phosphodiesterase superfamily, catalyzes the reaction β-Nicotinamide mononucleotide + ATP <=> Nicotinamide adenine dinucleotide (NAD) + PPi, representing the final step in the biosynthesis of NAD. NAD is a molecule that plays a fundamental role as a cofactor in cellular redox reactions. Thus β-Nicotinamide mononucleotide is an important metabolite for the maintenance of normal NAD biosynthesis. Circulating β-Nicotinamide mononucleotide levels may play an important role in regulating cell function in physiological and pathophysiological conditions.