Ca2+ ions are involved in the regulation of many diverse functions in animal and plant cells, e.g. muscle contraction, secretion of neurotransmitters, hormones and enzymes, fertilization of oocytes, and lymphocyte activation and proliferation. The intracellular Ca2+ concentration can be increased by different molecular mechanisms, such as Ca2+ influx from the extracellular space or Ca2+ release from intracellular Ca2+ stores. Release from intracellular Ca2+ stores is accomplished by the small molecular compounds D-myo-inositol 1,4,5-trisphosphate (InsP3), cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). This review will focus on the effects of cADPR in different cells and tissues, the mechanisms of cADPR-mediated Ca2+ release and Ca2+ entry, extracellular effects of cADPR, and the role of cADPR in a cell system studied in detail, human T-lymphocytes.