Resiency is a stage of graduate medical training and/or certification in a primary care or referral specialty. It is filled by a resident physician who has received a medical degree or diploma (MD, MBBS, MBChB, or DO) and in the United States 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.