NAD + metabolism and regulation are still incompletely defined in their features. Specifically, several compartmentation problems, both subcellular and intercellular, have emerged in the past years. A topological paradox was identified in the CD38/NAD + /Cyclic ADP-ribose system which represents a means for achieving regulation of intracellular Calcium levels ([Ca 2+ ] i and of Ca 2+ -mediated cell functions. CD38, an ectoenzyme featuring ADP-ribosyl cyclase (ADPRC) activity, is known to catalyze the generation of as many as eight Ca 2+ -active signal metabolites, including cADPR, ADPR, NAADP and three adenine homodinucleotides (Ap2A and two isomers thereof, designated P18 and P24). The first paradox concerns the ectocellular conversion of NAD + to cADPR and the intracellular activity of cADPR as a potent Ca 2+ mobilizer from stores expressing ryanodine receptors/channels on the endoplasmic/sarcoplasmic reticulum. This topological inconsistency was solved with the identification of hexameric Connexin 43 hemichannels (Cx43 HC) as an equilibrative transport system for NAD + , and of redundant equilibrative, and especially concentrative, transport systems for cADPR. Other transporters for NAADP + , P18 and P24 are still unknown. In addition, NAD + itself and some of its derivatives were recognized to be agonists of purinergic receptors, e.g., P2Y 11 and P2X 7 , thereby affecting the [Ca 2+ ] i levels either through protein kinase A-and phospholipase C-mediated pathways or through direct influx of extracellular Ca 2+ , respectively. Finally, recent findings demonstrate that the enzymes involved in NAD + biosynthesis and some of the enzymes involved in the multiple pathways of NAD + utilization localize to distinct subcellular compartments of the same cell and, surprisingly, also to different cell types of the same organism. Therefore, NAD + biosynthesis from several precursors, collectively defined Vitamin B3 forms, is at the same time an organismal and a cellular process, whose individual steps may occur in different cells/tissues/organs. Thus, a paracrine crosstalk is activated via the exchange of intermediate metabolites in biological fluids (e.g., Nicotinamide mononucleotide and Nicotinamide riboside), and the eventual NAD + biosynthesis occurs in selected cells able to utilize it for the display of diverse, fundamental cell functions. These are driven by NAD + -consuming enzymes, as ADPRCs, mono ADP-ribosyltransferases, poly(ADP-ribose) polymerases and different sirtuins (NAD + -dependent protein deacetylases).