Humidification during anesthesia is important to prevent adverse changes in the upper airways and possible pulmonary compromise. These changes may take place in less than 1 hour using dry nonhumidified anesthetic gases. Consequently, some method of humidification should be employed for all but the shortest of surgical procedures requiring general anesthesia. Methods of humidification include the anesthesia breathing system itself, passive humidification or conservation of moisture (the use of HMEs), and active humidification. The simplest system providing good levels of humidification and warming of anesthetic gases is the Circle system, which uses a fresh gas flow of less than 2 L/min. Its success can be further enhanced by use of a coaxial circuit. The minimum levels of water output or humidity required in the breathing circuit remain controversial. It may be preferable to have a gas with a lower temperature and higher relative humidity because a warmer gas that is less saturated may result in increased desiccation from the upper airways. Humidification for neonatal and pediatric patients requires special consideration of resistance, work of breathing, and dead space. Further work is necessary with regard to the use of HMEs in this population of patients undergoing general anesthesia.