Recent studies into the mechanisms of action of the Ca²⁺-mobilizing messenger NAADP (nicotinic acid–adenine dinucleotide phosphate) have demonstrated that a novel family of intracellular Ca²⁺-release channels termed TPCs (two-pore channels) are components of the NAADP receptor. TPCs appear to be exclusively localized to the endolysosomal system. These findings confirm previous pharmacological and biochemical studies suggesting that NAADP targets acidic Ca²⁺ stores rather than the endoplasmic reticulum, the major site of action of the other two principal Ca²⁺-mobilizing messengers, InsP₃ and cADPR (cADP-ribose). Studies of the messenger roles of NAADP and the function of TPCs highlight the novel role of lysosomes and other organelles of the endocytic pathway as messenger-regulated Ca²⁺ stores which also affects the regulation of the endolysosomal system.