Policymakers have long struggled to design a payment system that encourages plan and beneficiary participation in Medicare managed care without increasing Medicare spending relative to traditional fee-for-service. Under a 2006 reform, managed care firms ?bid? to provide the Medicare benefits package in county-level markets. Currently, little is known about how the bid system has influenced Medicare Advantage plan market structure, payments to plans, and Medicare beneficiary enrollment decisions. This project will use newly released panel data on Medicare Advantage markets from 2006 ? 2010 to assess trends in Medicare Advantage enrollment and plan payments over time.