Concern about certain contractual arrangements between health maintenance organizations (HMOs) and primary care physicians has led policymakers to consider curbing these arrangements; one law has already been passed. However, these arrangements are complex and their impact is neither obvious nor well understood. This article first presents a conceptual approach to understanding the relationship between HMOs and primary care physicians and discusses how they influence the locus of financial risk and managerial control. It then refines understanding of two critical dimensions (three-tiered HMOs and risk pools) by examining survey responses of 260 HMOs (representing over 50% of total HMO enrollment.) Results of the evaluation led to the conclusion that primary care physicians in three-tiered HMOs are sheltered from some of the financial incentives and contractual arrangements enacted by the HMO and that the reason for using risk pools may be due more to peer group effects or interaction with other incentives, rather than the direct financial implications of the risk pool on individual physicians. These concepts and observations have relevance for other types of health care systems in this country and elsewhere. Policymakers risk enacting misguided policies unless they understand the details of these arrangements.