The escalating number of emergency department (ED) visits, length of stay, and hospital overcrowding have been associated with an increasing number of critically ill patients cared for in the ED. Existing physiologic scoring systems have traditionally been used for outcome prediction, clinical research, quality of care analysis, and benchmarking in the intensive care unit (ICU) environment. However, there is limited experience with scoring systems in the ED, while early and aggressive intervention in critically ill patients in the ED is becoming increasingly important. Development and implementation of physiologic scoring systems specific to this setting is potentially useful in the early recognition and prognostication of illness severity. A few existing ICU physiologic scoring systems have been applied in the ED, with some success. Other ED specific scoring systems have been developed for various applications: recognition of patients at risk for infection; prediction of mortality after critical care transport; prediction of in-hospital mortality after admission; assessment of prehospital therapeutic efficacy; screening for severe acute respiratory syndrome; and prediction of pediatric hospital admission. Further efforts at developing unique physiologic assessment methodologies for use in the ED will improve quality of patient care, aid in resource allocation, improve prognostic accuracy, and objectively measure the impact of early intervention in the ED.