A medical residency is a post-graduate educational and clinical training program for physicians. Whereas medical school gives doctors a broad range of medical knowledge, basic clinical skills, and limited experience practicing the art of medicine, medical residency gives in-depth training within a specific branch of medicine, such as general surgery or ophthalmology. Medical residencies vary in duration from 3 years for primary care to upwards of 5 years for a more specialized field. During their medical residency training, doctors are called residents. The first year of residency is often called internship. Often the internship is combined with the residency program, but sometimes doctors do an internship in a general field of study and then complete a separate specialty residency program.

In the United States, doctors can obtain a general medical license after completing one year of internship. Many residents have medical licenses and do legally practice medicine ("moonlight") without supervision in settings such as urgent care centers and rural hospitals. However, in all residency-related medical settings, residents are supervised by attending physicians who must approve their decision-making.

Doctors must complete a residency in their speciality in order to become board certified. Board certification is not a requirement for the practice of medicine but it ensures the public that a doctor has completed training in a speciality and has passed an examination. In some specialities, recertification is required after a period of several years, to ensure that doctors stay up to date with new medical knowledge in their field.

Access to graduate medical training programs such as residencies is competitive. Applicants apply to programs, are selected for interview, and submit a rank-order list to a centralized matching service (currently the 'National Residency Matching Program' (NRMP). Residency programs submit a list of applicants in rank order as well. The two parties' lists are combined by an NRMP computer which (theoretically) creates optimal matches of residents to programs. On a certain day in March each year 'match day' these results are announced and are celebrated in hospitals and medical schools nationwide.

Medical residencies traditionally required brutally long hours of their trainees - classically, 36-hour shifts separated by 12 hours' rest, and 100+ hour weeks. The American public (though not some physicians trained under the same circumstances) is increasingly recognizing that such long hours are counter-productive; sleep deprivation increases rates of medical errors and motor vehicle accidents. The Accreditation Council for Graduate Medical Education (ACGME) has recently limited the number of work-hours to 80 hours weekly and the overnight call frequency to no more than one overnight every third day.

See also the subsection on training in the article about Physicians and Junior Doctors .