The internal medicine field is a specialty which concentrates on the diagnosis, management and non-surgical treatment of unusual or serious diseases. Specialists in this medical field are usually called "internists," but in some areas are also referred to as "physicians." Most internists specialize in a subcategory of internal medicine which focuses on problems with one particular organ inside the human body. Sub-specialties include: Cardiology (heart), Endocrinology (endocrine system), Gastroenterology (digestive system), Hematology (blood), Infectious Diseases (virus, bacteria, and parasites), Medical Oncology (cancer), Nephrology (kidneys), Pulmonology (lungs), Rheumatology (inflammation disorders), Adolescent Medicine, Clinical Cardiac Electrophysiology, Critical Care Medicine, Geriatric Medicine (aging), Interventional Cardiology, Sleep Medicine, Sports Medicine, Transplant Hepatology, Allergy and Immunology.

Internists study and train not only to diagnosis and treat severe chronic diseases, but also when more than one disease strikes an individual patient. Other organs and conditions internists may also treat are the skin, ears, substance abuse, mental health, nervous system and reproductive organs. The elderly population of the United States usually has an internist as their primary medical practitioner.

Internists must successfully complete extensive study and training. After completing a four-year undergraduate program at a university or college, the candidate must successfully enter and complete medical school, which usually takes an additional four to five years of study. Afterwards, the medical student enters a residency program which is approximately one to two years of supervised training. If the internist wishes to specialize in a subcategory of internal medicine as mentioned above, another three to ten years is spent in that particular specialty under a fellowship or internship program. If the specialty is more complex then more than ten years may be required.

Certification examinations must be taken by the internist. There are two organizations within the United States who certify internist specialists. They are the American Board of Internal Medicine and the American Osteopathic Board of Internal Medicine. For an internist specializing in allergy or immunology, the certifying board is the American Board of Allergy, Asthma and Immunology.

In the field of internal medicine, internists are also expected to successfully complete continuing education courses. Their certifications must be maintained by passing the "Maintenance of Certification Examinations." The Mayo Clinic offers an intensive course of study which prepares the internists to pass this exam. In addition, the Mayo Clinic offers other relevant courses for updating an internist’s skills and knowledge base. Some of these include, "Controversies in Women’s Health," which are medical conditions and treatments unique to women, "New Strategies for the Evaluation and Treatment of Congestive Heart Failure," "Selected Topics in Rheumatology," which highlights new advances in pathogenesis diagnosis and management of rheumatic diseases, "Pulmonary Hypertension Update," which helps the internist increase knowledge in pulmonary arterial hypertension from making an accurate diagnosis to determining the best treatment option for the patient, and "Genomics in Everyday Medical Practice," which discusses the importance of understanding the genomic basis of diseases and treatment effects, as internists must understand a patient’s genetic make-up to determine which therapies are the most effective.