Residency is a stage of postgraduate medical training certification in a primary care or referral specialty. It is filled by a resident physician who has received a medical degree (MD, MBBS, MBChB, or DO) and is composed almost entirely of the care of hospitalized or clinic patients, mostly with direct supervision by more senior physicians. A residency may follow the internship year or include the internship year as the first year of residency. The residency can also be followed by a fellowship, during which the physician is trained in a sub-specialty.

Whereas medical school teaches doctors a broad range of medical knowledge, basic clinical skills, and limited experience practicing medicine, medical residency gives in-depth training within a specific branch of medicine. A doctor may choose a residency in anesthesiology, sports medicine, dermatology, emergency medicine, family medicine, internal medicine, neurology, obstetrics and gynecology, pathology, pediatric medicine, psychiatry, physical medicine and rehabilitation, radiology, radiation oncology, or other specialties (ie. Surgery).

In the United States it leads to eligibility for board certification and membership/fellowship of several specialty colleges and academies. In Canada it leads to eligibility for Certification by and Fellowship of the Royal College of Physicians and Surgeons of Canada. In Australia and New Zealand it leads to eligibility for Fellowship of the Royal Australasian College of Physicians, the Royal Australasian College of Surgeons, or a number of similar bodies.