The 1990s are recognized as an era of heightened accountability for the quality and costs of medical care. Managed care and the development of methods to measure the quality and costs of care have ushered in this new era. The individual physician needs to understand the expectations of all of the players in this new medical delivery system, including the consumer, the purchaser, the payer, and other providers. The ability to measure performance and practice patterns requires attention to the validity of the data on which measures are based and the ability to account for those components of variations in performance that are due to the case mix and severity of patients being treated. This new accountability and responsibility for populations of patients necessitates a change in the mindset of the physician, from being an expert in managing each individual patient treated to becoming an expert manager of an entire population of patients.