Which temperature and humidity is optimal and can be recommended to the clinician? Some authors advocate the delivery of gas at body temperature and 100% relative humidity, which is equivalent to a water content of 44 mg/L [5,88,89]. They argue that energy neutrality is the best indicator of optimum humidity and that the intubated airway cannot be equated with the natural airway. Water loss as well as temperature and humidity gradients along the airway are necessary for mucociliary clearance and maintenance of the liquid layer of the airway epithelium, however . Theoretical considerations and long-lasting experience in clinical practice support a setting that mirrors physiologic conditions even in the intubated airway. Thus, saturated gas at a temperature of 330 degrees to 35 degrees C should be delivered to the airway threshold of ventilated neonates and infants. Heated humidifiers and some HMEs can comply with these conditions. With active humidification (primarily the condensation of water) over humidification or possible malfunctions must be kept in mind. The neonatologist must consider increase in deadspace, water-retention capability, leak around the tracheal tube, and the slight increase in airway resistance when using HMEs. HMEs should not be used during weaning from ventilatory support in babies who have a body weight less than 2500 g.