Residency is a stage of postgraduate medical training in North America which leads to eligibility for board certification in a primary care or referral specialty. Residency follows medical school, and may follow the internship year or include the internship year as the first year of residency.

Residencies as an opportunity for advanced training in a medical or surgical specialty evolved in the late 19th century from brief and less formal programs for extra training in a special area of interest. They became formalized and institutionalized for the principal specialties in the early twentieth century.

Residencies are traditionally hospital-based and in the middle of the twentieth century, residents would often live 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 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 intership, often served at different hospitals, and only a minority of physicians served them.

Many changes have occurred in postgraduate medical training in the last fifty years.

1. Nearly all doctors now serve a residency after graduation from medical school. In many states, full licensure for unrestricted practice is not available until graduation from a residency program. Residency is now considered desirable preparation even for primary care (what used to be called "general practice").
2. The internship has been subsumed into residency for most physicians. It is now uncommon for a physician to take a year of internship before entering a residency, and the first year of residency training is now considered equivalent to an internship for most legal purposes. Physicians who graduate from osteopathic medical schools (getting the D.O. degree instead of M.D.) are still encouraged and often required to take an internship before applying for residency.
3. The number of separate residencies has proliferated and there are now dozens. For many years the principal traditional residencies included internal medicine, gynecology, pediatrics, general surgery, ophthalmology, orthopedics, neurosurgery, otolarngology, urology, physiatry, and psychiatry. Family practice residencies have been available for many years.
4. Pay has increased and residents now make a wage which can support a family. Few residents live in hospital-supplied housing anymore, but unlike most attending physicians (that is, those who are not residents), they do not take call from home; they are usually expected to remain in the hospital for the entire shift.
5. Call hours have been greatly restricted. In July of 2003, strict rules went into effect for all residency programs in the US, known to residents as the "work hours rules". Among other things, these rules limited a resident to no more than 80 hours of work in a week, no more than 30 hours at a stretch (with no new patients in the last six), and call no more often than every third night. In-house call for most residents these days is typically one night in four; surgery and obstetrics residents are more likely to have one in three call. A few decades ago, in-house call every third night or every other night was the standard.
6. For many specialties an increasing proportion of the training time is spent in outpatient clinics rather than on inpatient care. Since in-house call is usually greatly reduced or absent on these outpatient rotations, this also contributes to the overall decrease in the total number of on-call hours.