Ambulatory care covers a wide range of health care services that are provided for patients who are not admitted overnight to a hospital. These services are performed at outpatient clinics, urgent care centers, emergency rooms, ambulatory or same-day surgery centers, diagnostic and imaging centers, primary care centers, community health centers, occupational health centers, mental health clinics, and group practices.

Ambulatory health care has grown tremendously since the early 1980s. Its growth has been driven by a desire for insurers and the United States government to control health care costs. In addition, advances in technology make many tests and surgeries that were formerly done as inpatient procedures in hospital settings safe to do in freestanding clinics or centers. The development of expensive specialty equipment such as magnetic resonance imaging (MRI) has lead to the establishment of specialty centers for diagnostic testing. Some health maintenance organizations...