Whereas medical school teaches physicians a broad range of medical knowledge, basic clinical skills, and supervised experience practicing medicine in a variety of fields, medical residency gives in-depth training within a specific branch of medicine. A physician may choose a residency in anesthesiology, ophthalmology, cardiothoracic surgery, dermatology, emergency medicine, family medicine, internal medicine, neurology, neurosurgery, obstetrics and gynecology, otolaryngology, pathology, pediatrics, plastic and reconstructive surgery, psychiatry, physical medicine and rehabilitation, radiology, radiation oncology, oral and maxillofacial surgery, general surgery, urology or other medical specialties.
A resident physician is more commonly referred to as a resident, senior house officer (in Commonwealth countries), or alternatively as a senior resident medical officer or house officer. Residents have graduated from an accredited medical school and hold a medical degree (MD, DO, MBBS, MBChB). Residents are, collectively, the house staff of a hospital. This term comes from the fact that resident physicians traditionally spend the majority of their training "in house," i.e., the hospital. Duration of residencies can range from three years to seven years, depending upon the program and specialty. A year in residency begins between late June and early July depending on the individual program, and ends one calendar year later. Depending on the number of years a specialty requires, the term junior resident may refer to residents that have not completed half their residency. Senior residents are residents in their final year of residency. Some residency programs refer to residents in their final year as chief residents (typically in surgical branches). Alternatively, a chief resident may describe a resident who has been selected to extend his or her residency by one year and organize the activities and training of the other residents (typically in internal medicine and pediatrics). If a physician finishes a residency and decides to further his or her education in a fellowship, he or she is referred to as a "fellow". Post-residency physicians are referred to as attending physicians, or consultants (in Commonwealth countries). However, the above nomenclature applies only in educational institutes in which the period of training is specified in advance. In privately owned, non-training hospitals, in certain countries, the above terminology may reflect the level of responsibility held by a physician rather than their level of education.
Residency as an opportunity for advanced training in a medical or surgical specialty evolved in the late 19th century from brief and informal programs for extra training in a special area of interest. The first formal residency programs were established by Sir William Osler and William Stewart Halsted at the Johns Hopkins Hospital. Residencies elsewhere then became formalized and institutionalized for the principal specialties in the early 20th century. But even mid-century, residency was not seen as necessary for general practice and only a minority of primary care physicians participated. By the end of the 20th century in North America though, very few new doctors went directly from medical school into independent, unsupervised medical practice, and more state and provincial governments began requiring one or more years of postgraduate training for medical licensure.
Residencies are traditionally hospital-based, and in the middle of the twentieth century, residents would often live (or "reside") in hospital-supplied housing. "Call" (night duty in the hospital) was sometimes as frequent as every second or third night for up to three years. Pay was minimal beyond room, board, and laundry services. It was assumed that most young men and women training as physicians had few obligations outside of medical training at that stage of their careers.
The first year of practical patient-care-oriented training after medical school has long been termed "internship." Even as late as the middle of the twentieth century, most physicians went into primary care practice after a year of internship. Residencies were separate from internship, often served at different hospitals, and only a minority of physicians did residencies.