In the past, most investigators failed to consider time relationships in their studies of circulatory problems. Because of this, data obtained in middle- or late-stage shock during organ failure are often presented as being characteristic of specific shock syndromes. Even "early" studies are not physiologically early, but instead have often come to mean early after ICU admission or early after life-threatening hypotensive events. The hypotensive episode represents decompensation of protective circulatory mechanisms, not the beginning of circulatory dysfunction. Early monitoring demonstrates that circulatory changes do not start with hypotension, but with the precipitating event, i.e., hemorrhage, trauma, surgery, or sepsis. When monitoring is started after hypotension, the first half of the problem is missed. It is, therefore, appropriate to focus on the earliest period of circulatory dysfunction with noninvasive methods to evaluate pathophysiology, to predict outcome, and to propose therapeutic protocols to improve outcome. Invasive monitoring is generally accepted as the "gold standard" for critically ill patients. The pulmonary artery flotation catheter has translated information to the bedside previously only available in cardiac catheterization laboratories, forever changing the way we treat ICU patients. Newer high-tech hardware and software innovations in the impedance method give more accurate and reliable cardiac index (CI) measurements that now satisfactorily agree with thermodilution in most clinical conditions. Minor disparities are more than made up for by the continuous online display of data. This impedance device, combined with pulse oximetry and transcutaneous oximetry, provides a feasible, noninvasive hemodynamic monitoring system that can be applied in a manner similar to electrocardiogram electrodes in the emergency department, operating room, ICU, hospital floors, and doctor's offices. More importantly, noninvasive monitoring provides a continuous, online, real-time display of hemodynamic data needed to titrate therapy rapidly and expeditiously. This is a major advantage, since therapy is more effective if given prophylactically or early and then titrated to optimal goals. Noninvasive monitoring provides a powerful method for objective evaluation of early, rapidly changing circulatory dynamics beginning with the precipitating event. This gives a new and different view of circulatory failure, exceeding the boundaries of our old concepts of shock based on blood pressure, subjective symptoms, and imprecise signs. Data of survivors revealed increased cardiac function (CI and oxygen delivery) shortly after surgery, trauma, and sepsis; this response is needed to meet the increased metabolic demands defined by the increased oxygen consumption. Nonsurvivors have limited responses to the added metabolic demands of external stressors. Therapy should augment naturally occurring compensations, but it must be given promptly within appropriate time limits.