Patients are admitted to critical care units from a variety of settings, including the emergency department, medical or surgical service, or operating room. Most critical care patients are acutely and severely ill, commonly with dysfunction or failure of more than one organ system. The initial assessment must be rapid and focus on real or potentially life- threatening processes that require immediate intervention. An example is the resuscitation of a patient with cardiopulmonary arrest. The pace of resuscitation is necessarily quick; physical examination may be restricted initially to the central nervous, cardiovascular, and respiratory systems, and interventions may be limited to the essential ABCs of airway, breathing, and circulation. Later, continuous electrocardiographic monitoring, measurement of blood pressure, and other standard procedures should start. In general, management of the critically ill patient should be based on an understanding of physiology and pathophysiology. Indeed, although the contributions of cell and molecular biology to critical care medicine are substantial, the critical care unit more resembles a physiology laboratory, since the effects of its interventions can be directly observed.