In the United States, the M.D. degree is the most common degree held by physicians and surgeons. Medical doctors are trained in accredited schools of medicine which are overseen by the Association of American Medical Colleges and the Liaison Committee on Medical Education, both independent boards of the American Medical Association, the AMA.

Admissions to medical schools in the United States is competitive, with less than one third of the approximately 35,000 applicants matriculating to a medical school. Before graduating from a medical school and achieving the degree of Medical Doctor, students have to pass the United States Medical Licensing Examination (USMLE) Step 1 and both the Clinical Knowledge and Clinical Skills parts of Step 2. The M.D. degree is typically earned in four years. Following the awarding of the M.D., physicians who wish to practice in the United States are required to complete at least one internship year (PGY-1) and pass the USMLE Step 3. Most, in order to receive Board Eligible or Board Accredited status in a specialty of medicine such as general surgery or internal medicine, then undergo additional specialized training in the form of a residency. Those who wish to further specialize in areas such as cardiology or interventional radiology then complete a fellowship. Depending upon the physician's chosen field, residencies and fellowships involve an additional three to eight years of training after obtaining the M.D. This can be lengthened with additional research years, which can last one, two, or more years.

In Canada, the M.D. is the basic medical degree required to practice medicine; as well, in most provinces, the provincial college of physicians and surgeons grants practice rights to American-trained osteopathic physicians. At McGill University in Montreal, M.D.C.M. (Medicinae Doctorem et Chirurgiae Magistrum) degrees are awarded.

Though the M.D. degree is a professional doctorate, and not a research doctorate, many holders of the M.D. degree conduct clinical and basic scientific research and publish in peer-reviewed journals during training and after graduation. Some M.D.s choose a research career and receive funding from the NIH as well as other sources such as the Howard Hughes Medical Institute. A few even go on to become Nobel Laureates.[2]

The abbreviation "M.D." is frequently used post-nominally in the US, being put after the name as a title; however, it is also used on its own in informal writing, as an abbreviation for "medical doctor." It is one of the most recognized degrees in the general public and the media, and sometimes incorporated into the titles of television shows such as House MD, or Doogie Howser, M.D..