Many critically ill patients are remaining in the emergency department for extended periods of time, and delays in diagnosis and/or therapy may increase patient morbidity and mortality. All emergency physicians use monitoring modalities in critically ill patients to detect early cardiovascular compromise and impaired oxygen delivery before disastrous collapse occurs. The authors hope the discussion in this article regarding the monitoring of oxygenation, ventilation, arterial perfusion pressure, intravascular volume, markers of tissue hypoxia, and cardiac output will help the EP provide optimal care for this complicated patient population.