An emergency department (ED), also known as an accident & emergency department A&E), emergency room (ER), emergency ward (EW) or casualty department, is a medical treatment facility or section specializing in emergency medicine, the acute care of patients who present without prior appointment; either by their own means or by that of an ambulance.

Due to the unplanned nature of patient attendance, the department must provide initial treatment for a broad spectrum of illnesses and injuries, some of which may be life-threatening and require immediate attention. In some countries, emergency departments have become important entry points for those without other means of access to medical care.