Internal medicine is the branch and specialty of medicine concerning the diagnosis and nonsurgical treatment of diseases in adults, especially of internal organs. Doctors of internal medicine, also called "internists", are required to have included in their medical schooling and postgraduate training at least three years dedicated to learning how to prevent, diagnose, and treat diseases that affect adults. Internists are sometimes referred to as the "doctor's doctor,"

Internists hold either an allopathic (MD,MBBS,MBChB,etc) or osteopathic (DO) degree. They are not to be confused with "Medical Interns," who are either physicians in their first year of residency training (in countries like the USA), or last-year medical students (in countries like Colombia). Although Internists may act as primary care physicians, they are not "family physicians," "family practitioners," or "general practitioners" (whose training in certain countries includes the medical care of children, and may include surgery, obstetrics and pediatrics).