In the past mechanical ventilation always mimicked the tidal volumes and ventilatory frequencies of normal breathing. Recently, there has been great interest in techniques that use rapid rates (60 to 3,000 per minute) and tidal volumes approximating dead space. These techniques are known collectively as high-frequency ventilation, although they differ in circuit design, use, potential complications, and mechanism of gas transport. High-frequency ventilation can be divided into four categories: (1) high-frequency positive pressure ventilation, (2) high-frequency jet ventilation, (3) high-frequency oscillatory ventilation and high-frequency flow interruption, and (4) high-frequency chest wall oscillation. In this review we discuss the similarities and differences of these high-frequency techniques, their clinical applications, and some physiological mechanisms involved in gas transport.