We survey the historical development of scientific knowledge surrounding Vitamin B3, and describe the active metabolite forms of Vitamin B3, the pyridine dinucleotides NAD⁺ and NADP⁺ which are essential to cellular processes of energy metabolism, cell protection and biosynthesis. The study of NAD⁺ has become reinvigorated by new understandings that dynamics within NAD⁺ metabolism trigger major signaling processes coupled to effectors (sirtuins, PARPs, and CD38) that reprogram cellular metabolism using NAD⁺ as an effector substrate. Cellular adaptations include stimulation of mitochondrial biogenesis, a process fundamental to adjusting cellular and tissue physiology to reduced nutrient availability and/or increased energy demand. Several mammalian metabolic pathways converge to NAD⁺, including tryptophan-derived de novo pathways, nicotinamide salvage pathways, nicotinic acid salvage and nucleoside salvage pathways incorporating nicotinamide riboside and nicotinic acid riboside. Key discoveries highlight a therapeutic potential for targeting NAD⁺ biosynthetic pathways for treatment of human diseases. A recent emergence of understanding that NAD⁺ homeostasis is vulnerable to aging and disease processes has stimulated testing to determine if replenishment or augmentation of cellular or tissue NAD⁺ can have ameliorative effects on aging or disease phenotypes. This experimental approach has provided several proofs of concept successes demonstrating that replenishment or augmentation of NAD⁺ concentrations can provide ameliorative or curative benefits. Thus NAD⁺ metabolic pathways can provide key biomarkers and parameters for assessing and modulating organism health.