This paper discusses new techniques for measuring cardiac output noninvasively, and their applications in critically ill and injured patients. Determination of cardiac output has become an integral part of clinical critical care. Since the availability of pulmonary artery catheters in the mid 1 970s, much of the therapy and diagnosis in the intensive care unit is directed by the measurements obtained through invasive hemodynamic monitoring. Costs and complications associated with invasive hemodynamic monitoring are a major concern, limiting the availability of this information to the intensive care unit and the operating room. Advances in technology and understanding of physiology offer the opportunity to measure cardiac output noninvasively. A noninvasive approach will reduce costs and complications and provide the capacity to obtain information wherever the patient may be located.