Nicotinamide adenine dinucleotide (NAD(+)) plays a critical role in regulating numerous biological and physiological pathways including metabolism, inflammation, cancer, and aging in mammals. Here we describe a highly quantitative method with reverse-phase high-performance liquid chromatography (HPLC) for the determination of NAD(+) levels in cells and tissues. This methodology provides accurate, reliable, and reproducible results of NAD(+) measurement, which enables us to analyze various pathophysiological changes in NAD(+) levels in vitro and in vivo.