The American College of Physicians (ACP) is a national organization of octors of internal medicine (internists), physicians who specialize in the prevention, detection and treatment of illnesses in adults. With more than 119,000 members, ACP is the largest medical-specialty organization and second-largest physician group in the United States of America. ACP provides information and advocacy for its members as they practice internal medicine and related subspecialties. ACP members are also involved in medical education, research, and administration. There are seven classes of membership, including medical students, residents, and fellows.