Doctors of internal medicine ("internists") focus on adult medicine and have had special study and training focusing on the prevention and treatment of adult diseases. At least three of their seven or more years of medical school and postgraduate training are dedicated to learning how to prevent, diagnose, and treat diseases that affect adults.

Doctors of internal medicine may be referred to by several terms, including "internists," "general internists" and "doctors of internal medicine." They are not to be confused with "interns," who are doctors in their first year of residency training. Although internists may act as primary care physicians, they are not "family physicians," "family practitioners," or "general practitioners," whose training is not solely concentrated on adults and may include surgery, obstetrics and pediatrics. General internists practice medicine from a primary care perspective but they can treat and manage 90 plus % of all ailments.

Internal medicine subspecialists may also practice general internal medicine, but usually focus their practice on their particular subspecialty (like cardiology or pulmonology).