This chapter describes the discovery of cyclic adenosine diphosphate–ribose (cADPR) as a novel endogenous Ca2+-mobilizing agent, and the way by which it fulfills most of the criteria necessary for it to be considered a second messenger. The enzymatic pathways for the synthesis and degradation of the metabolite are also summarized. The metabolic pathway of cADPR consists of synthesis from nicotinamide adenine dinucleotide (NAD+) by ADP–ribosyl cyclase and degradation by the cADPR hydrolase to ADP-ribose. CD38-like bifunctional enzymes are responsible for regulating the cellular concentration of cADPR. CD38 is an ecto-enzyme catalyzing the synthesis and the degradation of cADPR raises the possibility that cADPR may have extracellular functions. The properties of its intracellular receptor and the mechanism of its Ca2+-mobilizing activity are discussed. The physiological roles of cADPR in two specific cellular systems are reviewed in the chapter: the sea urchin egg, an invertebrate cell; and the pancreatic β cell, a mammalian system.