During high frequency oscillation (HFO) small volumes are delivered at frequencies up to 40 Hz; both inspiration and expiration are active. Oxygenation is controlled by the mean airway pressure level and carbon dioxide elimination by the delivered volume and, less so, by frequency. In some infants the delivered volume can be 50%-100% of the dead space. Measurements during HFO demonstrate oscillator performance can be impaired at high frequencies. HFO may be a useful alternative to extra corporeal membrane oxygenation in infants with severe respiratory failure, but its role in reducing bronchopulmonary dysplasia remains controversial.