Humidification of inspired gases during mechanical ventilation remains a standard of care. Optimal humidity is an elusive target and is not clearly defined in the literature. The choice of a humidification device cannot be made solely on the basis of moisture output, however. The clinician must consider the effects of the device on gas exchange and spontaneous breath-ing. The author's group has used the data reviewed here to modify their previous algorithm for choosing a humidification device (see Fig. 2). Humidification requirements for noninvasive ventilation need further study.