At its core, emergency medicine is the medical specialty concerned with the evaluation, management and treatment of patients with unexpected illness or injury. As opposed to most other specialties, emergency medicine has considerable involvement in working outside formal medical facilities; emergency physicians are often medical directors, and sometimes overall chiefs, of emergency medical services from first responders, through various levels of emergency medical technician including advanced paramedics, to the operation of field hospitals. An emergency physician, over a radio link, may guide minimally trained people in dealing with critical situations in remote areas, with no medical facilities in reasonable range.
Effective emergency medicine is one of the most demanding specialties in terms of physician flexibility, as well as flexibility within a triage-defined medical delivery system. Formal emergency medicine departments, but not specialization, usually is considered to have started in 1961. The specialty was formalized in 1968, when John Wiegenstein and seven colleagues founded the American College of Emergency Physicians (ACEP). ACEP's first Scientific Assembly was held in 1969. 
In the United States, emergency medicine is heavily influenced by the Emergency Medical Treatment and Active Labor Act. On the one hand, EMTALA has the desirable effect of ensuring that patients in acute danger will be evaluated and treated without first establishing the ability to pay. On the other hand, EMTALA makes the emergency room, which is the most expensive way to deliver care, the de facto primary care provider for a substantial portion of the uninsured.
Graduate medical education
A residency in emergency medicine is normally three years long. Some programs include joint training in pediatrics, pediatric emergency medicine, or family practice. In 2005, there were 135 allopathic and 29 osteopathic emergency medicine residency programs, which collectively accept about 1,500 new residents each year.
In the United States, it took some time for the two to agree, but now board-eligible programs may be under either the American Board of Emergency Medicine (ABEM) or the American Osteopathic Board of Emergency Medicine (AOBEM).