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A fellowship is the period of medical training in the United States and Canada that a physician or dentist may undertake after completing a specialty training program (residency). During this time (usually more than one year), the physician is known as a fellow. Fellows are capable of acting as attending physician or consultant physician in the generalist field in which they were trained, such as internal medicine or pediatrics. After completing a fellowship in the relevant sub-specialty, the physician is permitted to practice without direct supervision by other physicians in that sub-specialty, such as cardiology or oncology.
In the USA, the majority of fellowships are accredited by the ACGME Accreditation Council for Graduate Medical Education. There are a few programs that are not accredited as well, and are actually well received given that it is more important to be board certified for the primary specialty as a physician, and fellowship quality is often more based on research productivity. 
The following are organized based on specialty required for the fellowship.
Internal Medicine or Pediatrics
- Infectious disease
- Critical care medicine
Over recent years, there has been an increasing number of integrated programs, allowing for faster completion of long residencies (eg thoracic surgery integrated lasts 6 years instead of the 8 it would otherwise take).
- Maternal Fetal Medicine
- Minimally Invasive Gyn Surgery
- Pediatric and Adolescent Gyn
- Reproductive Endocrinology
- Sports Medicine
- Foot and Ankle
- Joint replacement
- Female Pelvic Medicine and Reconstructive Surgery
- Clinical Informatics
- Hospice and Palliative Medicine
- Interventional Radiology
- Sleep medicine
- Sports medicine
- Transplant hepatology
There are a number of programs offering a combined fellowship, training in two or more sub-specialties as part of a single program.
- Pulmonary/Critical Care: this type of program is more common than Pulmonary Disease (non-combination) programs. As of 2007, there were 130 ACGME-accredited combined Pulmonary/Critical Care programs while only 25 programs for Pulmonary Disease alone.
- Hematology/Oncology: as of 2005, there were 125 ACGME-accredited programs for Hematology-Oncology, while only 12 programs for Hematology alone and 18 for Oncology alone.
- Geriatrics/Oncology: the American Board of Internal Medicine approved a 3-year combined fellowship training program in medical oncology and geriatrics. The John A. Hartford Foundation initially funded 10 institutions for this type of training.
- Medical intern
- Medical specialty
- Physician specialty codes
- Society of General Internal Medicine
- Residency (medicine)
- Attending physician