A primary aspect of cardiovascular support of the critically ill patient is the titration of cardiopulmonary therapies based on the baseline cardiopulmonary status and the subsequent physiologic response. Implicit in this paradigm is the monitoring of the processes being titrated. The degree to which a specific physiologic variable, such as mean arterial pressure or arterial oxygen saturation, needs to be assessed is a function of the therapy used, the stability of the patient, the relation among the variables defining the hemodynamic profile, and the ability of the support staff to remain in close attendance at the bedside. In an otherwise stable patient in heart failure being treated with mild afterload reduction and diuretics, periodic measures of heart rate, urine output, and daily measures of body weight are all that are reasonably needed to titrate therapy. However, in the management of a patient with cardiogenic shock with pulmonary edema and respiratory failure, continuous measures of mean arterial pressure, left ventricular filling pressure, cardiac output, arterial oxygen content, and end-organ function may be necessary as more potent and risky therapies are used. How, then, does one arrive at the correct formula to prescribe appropriate physiologic monitoring for the patient in the intensive care unit setting? To a large extent this is unknown, primarily because the utility of monitoring techniques to diagnose pathophysiologic processes and the resultant effect of therapy to reverse it is not known for most of the diseases treated in the intensive care unit. Few monitoring techniques have progressed through a logical progression of development to their present level of use. Thus, their use in the management of the critically ill patient cannot be vigorously defended, except under specific conditions.