The recent discovery of genes encoding the mitochondrial calcium (Ca2+) uniporter has revealed new opportunities for studying how abnormal Ca2+ signals cause disease. Ca2+ transport across the mitochondrial inner membrane is highly regulated, and the uniporter is the channel that acts as a major portal for Ca2+ influx. Low amounts of mitochondrial Ca2+ can boost ATP synthesis, but excess amounts, such as following cytoplasmic Ca2+ overload in heart failure, triggers mitochondrial failure and cell death. In fact, precisely because mitochondrial Ca2+ transport is so tightly regulated, a fundamental understanding of how the uniporter functions is necessary. Two key uniporter features allow Ca2+ influx without mitochondrial damage during normal physiology. First, the channel is significantly more selective than other known Ca2+ channels. This prevents the permeation of other ions and uncoupling of the electrochemical gradient. Second, the uniporter becomes active at only high Ca2+ concentrations, preventing a resting leak of cytoplasmic Ca2+ itself. Now possessing the identities of the various proteins forming the uniporter, we can proceed with efforts to define the molecular determinants of permeation, selectivity and Ca2+-regulation. (C) 2014 Elsevier Inc. All rights reserved.