The use of the pulmonary artery (PA) catheter has become a central part of the management of critically ill patients in modern intensive care units. The PA catheter provides a remarkable amount of information that continues to increase with ongoing technological advances. In modern intensive care units, measurement of the pulmonary arterial wedge pressure (PAWP) is perhaps the most common and most important indication for inserting PA catheters in critically ill patients. The interpretation of the measured PAWP in such patients is also the most controversial aspect of the use of PA catheters, however. The most important use of PAWP is to estimate (not measure) two parameters: (1) the hydrostatic pressure gradient for pulmonary edema formation and (2) the left ventricular end-diastolic volume (LVEDV) or preload. PAWP does not directly measure either of these two parameters. When properly acquired, mean PAWP, taken at end-expiration, generally provides a reliable measure (a minimum estimate) of the pulmonary microvascular hydrostatic pressure, which is the major determinant of fluid flux from the pulmonary microvasculature. Similarly, when properly acquired, end-diastolic PAWP, taken at end-expiration, generally provides a reliable estimate of left ventricular end-diastolic pressure (LVEDP), but importantly this value may not reliably reflect LVEDV, or preload, if left ventricular (LV) compliance or juxtacardiac pressure are abnormal.