Internal medicine or general internal medicine (in Commonwealth nations) is the medical specialty dealing with the prevention, diagnosis, and treatment of internal diseases. Doctors specializing in internal medicine are called internists, or physicians (without a modifier) in Commonwealth nations. Internists are medical specialists that are skilled in the management of patients who have undifferentiated or multi-system disease processes. Internists care for hospitalized (inpatient) and ambulatory (outpatient) patients and may play a major role in teaching and research. Internists are qualified physicians with postgraduate training in internal medicine and should not be confused with "interns", a term for doctors in their first year of residency training.

Internal medicine and family medicine are often confused as equivalent in the United States and 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. The certification process along with the list of possible sub-specialties vary around the world.

Internal medicine is also a specialty within clinical pharmacy and veterinary medicine.