End-tidal carbon dioxide (CO2) monitoring is useful in the prehospital setting, emergency department, intensive care unit, and operating room. Capnography provides valuable, timely information about the function of both the cardiovascular and respiratory systems. End-tidal CO2 monitoring is the single most useful method in confirming endotracheal tube position. It also provides information about dead space, cardiac output, and airway resistance. A thorough understanding of cardiopulmonary physiology and the technical nuances of capnometry is required for its optimal use in children. This review examines the basic physiology pertinent to end-tidal CO2 monitoring, its clinical applications, and evidence supporting its use in infants and children.