Increasingly, health care providers are being evaluated and held accountable for their patients' outcomes, ranging from the costs to service consumption to death. To be meaningful, the outcomes under scrutiny must be important to patients or to the health care system as a whole, relatively common, and linked temporally and causally to the care provided. In addition, outcomes findings should be adjusted for patient risk factors, with the goal of accounting for pertinent clinical characteristics before drawing inferences about the effectiveness or quality of care. Risk adjustment "levels the playing field" in comparing outcomes across providers. Although this concept is straightforward, performing clinically credible risk adjustment is difficult, especially given the widespread data constraints. In this article, I review the major issues involved in performing risk adjustment for health care outcomes studies.