Internal medicine is the branch of medicine that deals with treating the whole patient. An internal medicine physician, also called an internist, can treat many illnesses and conditions, and is skilled in treating a patient who has several illnesses or disorders at the same time. An internist, who can be a primary care physician, emphasizes disease prevention and wellness, but can treat problems of the eyes, ears, skin, nervous system and reproductive organs, along with mental health or substance abuse issues.

Internists are sometimes referred to as the "doctor's doctor," because they are often called upon to act as consultants to other physicians to help solve puzzling diagnostic problems.