A fellowship is the period of medical training in the United States and Canada that a physician, dentist, or veterinarian may undertake after completing a specialty training program (residency). During this time (usually more than one year), the trainee is known as a fellow or fellow physician. Fellows are capable of acting as an attending physician or a consultant physician in the specialist 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 US, the majority of fellowships are accredited by the Accreditation Council for Graduate Medical Education ("ACGME") or, to a lesser extent, the American Board of Physician Specialties in select states. There are fellowship programs that are not ACGME accredited, yet are well received, given the importance of being a Board-Certified Physician in a primary specialty, where a Fellowship is often more based on demand and research productivity.

In general, ACGME accredited programs require completion of ACGME-accredited, RCPSC-accredited or CFPC- accredited residency program, however, exceptions for an ACGME-International- accredited residency programs and non-ACGME-accredited residency programs are possible. International medical graduates must be ECFMG certified. Some fellowship specialties require participation in special matching programs such as Specialties Matching Service® (SMS®) or SF Match.

There are a number of programs offering a combined fellowship, training in two or more sub-specialties as part of a single program.