 High frequency chest wall oscillation HFCWO has been shown to increase tracheal mucous clearance compared with control. These observations led to the development of The Vest which is a non-stretchable jacket connected to an air-pulse generator and worn by the patient over the chest wall. The generator rapidly inflates and deflates The Vest which gently compresses and releases the chest wall between 5 and 20 times per second. This generates mini-coughs which are said to dislodge mucus from the bronchial walls and to facilitate its movement up the airways. In addition The Vest has been shown to reduce the viscosity of mucus and this should further enhance mucous clearance. The technique has, for many years, been widely used in the United States of America as an alternative to the European airway clearance regimens of the active cycle of breathing techniques, autogenic drainage, positive expiratory pressure and oscillating positive expiratory pressure, but The Vest has only recently been registered for use in Europe. It is important that the airway clearance regimen of The Vest be compared with the alternative airway clearance regimens widely used in Europe. 