Internal medicine or general internal medicine (in Commonwealth nations) is the medical specialty dealing with the prevention, diagnosis, and treatment of internal diseases. Physicians specializing in internal medicine are called internists, or physicians (without a modifier) in Commonwealth nations. Internists are skilled in the management of patients who have undifferentiated or multi-system disease processes. Internists care for hospitalized and ambulatory patients and may play a major role in teaching and research. Internal medicine and family medicine are often confused as equivalent in the Commonwealth nations (see below).

Because internal medicine patients are often seriously ill or require complex investigations, internists do much of their work in hospitals. Internists often have subspecialty interests in diseases affecting particular organs or organ systems.