The prominence of physicians in highly interdependent medical systems confers tremendous power on them, individually and as a profession. With this power comes an ethical responsibility to be deeply concerned about medical systems. Examples of medical systems include the process of treating patients with diabetes; a hospital; the development and testing of new medical procedures; and a medical practice, including locations of care, billing, and collection of fees for medical care. The physician who is willing to learn about the nature of systems, how to control them, and how to improve them can significantly influence medical systems. Many persons in health care organizations identify strongly with their individual profession or department. Management structures, professional organizations, and methods of billing for services reinforce these divisions. This fragmented environment allows the structure of medical systems to evolve piecemeal from the various actions and points of view of physicians, nurses, administrators, patients, and payers. Improvement results from new structures that are purposefully designed. To achieve improvement, people must look beyond their own professional or organizational identities and see themselves as part of the larger system. Even a rudimentary understanding of the structures and dynamics of systems combined with clinical knowledge can equip a physician to collaborate with colleagues to diagnose faults of a system and design remedies. This paper explores the nature of medical systems and develops ideas their proper application to medicine and the activities of physicians.