Bedside measurement of the pulmonary artery occlusion pressure (wedge, Pw) has become routine in the practice of critical care. Although Pw is a potentially useful index of left ventricular filling pressure and pulmonary vascular congestion, an invalid or misinterpreted value can prompt inappropriate clinical decisions. To avoid error Pw must be measured precisely and interpreted carefully, with consideration given to disordered cardiopulmonary mechanics and to the objectives of therapy. This review focuses on the clinical physiologic aspects of the wedge pressure determination, the technical principles of measurement, and the practical application of the information Pw provides.