Physician payment has been one of the most rapidly evolving aspects of the health care system. Among public payers, the Medicare program changed from determining physician payments by calculating prevailing rates charged in each locality to establishing a resource-based fee schedule, with conversion factors updated through an expenditure-target mechanism. Medicare also shifted from leaving physicians and beneficiaries to work out any additional payments beyond the program's allowed charge (balance bills) to a participating-physician option and tight limits on balance billing for unassigned claims.