Mounting evidence attests to the paramount importance of the non-redox NAD functions. Indeed, NAD homeostasis is related to the free radicals-mediated production of reactive oxygen species responsible for irreversible cellular damage in infectious disease, diabetes, inflammatory syndromes, neurodegeneration and cancer. Because the cellular redox status depends on both the absolute concentration of pyridine dinucleotides and their respective ratios of oxidized and reduced forms (i.e., NAD/NADH and NADP/NADPH), it is conceivable that an altered regulation of the synthesis and degradation of NAD impairs the cell redox state and likely contributes to the mechanisms underlying the pathogenesis of the above mentioned diseases. Taking into account the recent appearance in the literature of comprehensive reviews covering different aspects of the significance of NAD metabolism, with particular attention to the enzymes involved in NAD cleavage, this monograph includes the most recent results on NAD biosynthesis in mammals and humans. Due to recent findings on nicotinamide riboside as a nutrient, its inclusion under "niacins" is proposed. Here, the enzymes involved in the de novo and reutilization pathways are overviewed.