During the early twentieth century American physicians considered several different ways to identify medical specialists. The first autonomous specialty board was incorporated in 1924, and over the next few decades the board system became the accepted system for credentialing specialists. In 1936 the American Board of Internal Medicine (ABIM) became the twelfth specialty board. It was intended to recognize only a few outstanding internists, for the ABIM’s founders believed that general practitioners should continue to deliver the vast majority of care. However, World War II greatly increased the importance of the ABIM and of the specialty boards system. The army medical services emphasized the importance of board certification, thus encouraging physicians to receive residency training and to sit for a specialty board. After the war, the expanding VA hospital system provided them a place to do so. Specialty training and board certification became the norm, not the exception. Also, the national exigencies of World War II forced Congress to devise a new system to distribute federal funds for biomedical research to universities. That system, continued as the National Institutes of Health, supported the subsequent growth of internal medicine in general, as well as its subspecialties. Both the ABIM and the other specialty boards were formed at a particular time and place in response to the specific interests of individuals and organizations, and their histories reflect both the ideals of their founders and the long-term structural effects of war.