Despite considerable data, little is certain about changes in breathing during sleep, let alone possible mechanisms for these changes. This article reviews methodologic problems, including the definition of the normal subject, the definition of the sleep state, and the effects of instrumentation, and discusses data on respiration during REM and NREM sleep in normal humans. Although the evidence is incomplete, respiration during sleep appears fragile, prone to instability, to upper airway obstruction, to hypoventilation, and to ventilation-perfusion mismatch, jeopardizing the homeostatic function of CO2 output and O2 uptake.