Many interventions of varying complexity, intensity, and duration have been applied in an attempt to alleviate or control ED overcrowding. While most seemed to reduce overcrowding, it is difficult to determine the relative value of these interventions, and the lack of comparison studies makes it impossible to say which ones work best. These results suggest that efforts to address overcrowding at an institutional level should be encouraged and monitored, because they have a high chance of success. There is a need for more studies on the specific effects of interventions, and how they might affect the quality of care and patient outcomes. Better reporting is needed on setting characteristics, study design, treatment description, and outcome measures to improve the process of synthesizing evidence on interventions to reduce overcrowding.