Abstract. Objective: To describe the definition, extent, and factors associated with overcrowding in emergency departments (EDs) in the United States as perceived by ED directors. Methods: Surveys were mailed to a random sample of EDs in all 50 states. Questions included ED census, frequency, impact, and determination of overcrowding. Respondents were asked to rank perceived causes using a five-point Likert scale. Results: Of 836 directors surveyed, 575 (69%) responded, and 525 (91%) reported overcrowding as a problem. Common definitions of overcrowding (>70%) included: patients in hallways, all ED beds occupied, full waiting rooms >6 hours/day, and acutely ill patients who wait >60 minutes to see a physician. Overcrowding situations were similar in academic EDs (94%) and private hospital EDs (91%). Emergency departments serving populations ≤250,000 had less severe overcrowding (87%) than EDs serving larger areas (96%). Overcrowding occurred most often several times per week (53%), but 39% of EDs reported daily overcrowding. On a 1-5 scale (±SD), causes of overcrowding included high patient acuity (4.3 ± 0.9), hospital bed shortage (4.2 ± 1.1), high ED patient volume (3.8 ± 1.2), radiology and lab delays (3.3 ± 1.2), and insufficient ED space (3.3 ± 1.3). Thirty-three percent reported that a few patients had actual poor outcomes as a result of overcrowding. Conclusions: Episodic, but frequent, overcrowding is a significant problem in academic, county, and private hospital EDs in urban and rural settings. Its causes are complex and multifactorial.