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

• Adolescent medicine
• Cardiology, dealing with disorders of the heart and blood vessels
• Clinical cardiac electrophysiology
• Critical care medicine
• Endocrinology, dealing with disorders of the endocrine system and its specific secretions called hormones
• Gastroenterology, concerned with the field of digestive diseases
• Diabetes care medicine
• Geriatric medicine
• Hematology, is concerned with blood, the blood-forming organs, and its disorders.
• Hospital medicine
• Infectious disease, concerned with disease caused by a biological agent such as a virus, bacterium, or parasite
• Interventional cardiology
• Medical oncology, dealing with the chemotherapeutic (chemical) treatment of cancer
• Nephrology, dealing with the study of the function and diseases of the kidney
• Pulmonology, dealing with diseases of the lungs and the respiratory tract
• Rheumatology, devoted to the diagnosis and therapy of rheumatic diseases.
• Sleep medicine
• Sports medicine
• Transplant hepatology

Internists may also specialize in “allergy” and “immunology.” The American Board of Allergy, Asthma, and Immunology is a conjoint board between internal medicine and pediatrics