Extraordinary growth in managed care arrangements over the past decade has been both widely praised and criticized. Proponents and critics agree that the nature of medical practice is being profoundly altered by this growth, even if they cannot articulate the direction and consequences of this change. We explore the roots of this uncertainty by examining the available evidence on critical features of the arrangements managed care plans currently have with affiliated physicians. Our approach is to review and synthesize the literature in several key substantive areas from a broad range of sources. We found that existing knowledge is dated, derived form a limited subset of plans, inattentive to important structural differences between plans, and responsive to a very narrow set of issues poorly reflecting the range of medical practice and change introduced by managed care. We highlight key questions of interest and the knowledge gaps critical to address so that policy and management decisions can both reflect and be informed on these issues that define the arrangements managed care plans make with physicians and ultimately influence medical practice.