ADP-ribosyI cyclase and CD38 are multi-functional enzymes involved in the synthesis and metabolism of two structurally and functionally distinct Ca²⁺ messengers, cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). cADPR is produced by cyclization of NAD at the N1 of the purine ring, while NAADP is synthesized by the exchange of the nicotinamide group of NADP with nicotinic acid. Both enzymes also cyclize the guanine analog of NAD, NGD, to produce the fluorescent albeit inactive analog, cyclic GDP- ribose. Calcium mobilization by cADPR and NAADP occurs via novel mechanisms that are totally independent of each other as well as that mediated by inositol triphosphate (IP₃). The cyclase and CD38 are homologous with 34% sequence identity and an additional 42% conserved substitutions. Using X-ray crystallography and site-directed mutagenesis the active site of the cyclase has been identified and characterized. Site-directed mutagenesis has also been used to define the catalytic site of CD38. This article encompasses recent developments from on-going studies on the structure-function relationships of these two enzymes.