There are two types of HMOs: prepaid group practices (PGPs) and individual practice associations (IP As). Because of rapid change in the HMO industry, the academic literature, which is based primarily on data from the 1970s, is dated in several ways. The literature has focused on PGPs, but IP As are growing three times faster than PGPs and are likely to dominate the HMO industry in the near future. The literature indicates that a small proportion of an IPA physician's practice is capitated, but such practices often are as much as one-third capitated. And while the literature indicates that IPA physicians are rarely given financial incentives to control cost, such incentives are now common. In this decade IPAs have cut their hospital days per thousand enrollees. Furthermore, IPAs in which individual physicians or medical groups bear some of the financial risk of hospital costs appear to have the same rate of hospital days as PGPs. Additional research is especially needed on cost and quality of care in modern IPAs.