There have been influential advocates for financing and organizing health care in the United States and England based on the model of integrated health care delivery systems (IHCDSs). Despite good evidence that a few IHCDSs provide high-quality health care economically, such organizations are rare and localized in a few market areas in the United States and are absent in the English National Health Service (NHS). The explanation of why this is so includes various contributory factors: the way the development of the medical profession in each country pursued specialization; the division in British medicine between general practitioners and specialists; and the characteristics that we identify of established successful IHCDSs, which created formidable barriers to entry for a new IHCDS. This explains why currently the most promising organizational developments in U.S. health care are hybrids resulting from vertical integration. In England government policies of an "internal market," as adopted in the 1990s and currently, were and are based on a purchaser-provider split with the objectives that providers would compete and be funded by a system in which "money follows the patient." These policies recognize the division in British medicine, which also means that it is difficult to implement a reorganized English NHS based on high-performing IHCDSs.