Beginning in 2006 the Medicare Prescription Drug, Improvement, and Modernization Act (MMA) will offer pharmacy benefits to forty-two million Medicare beneficiaries nationwide. In a 2003 national survey of Medicare beneficiaries age sixty-five and older, more than one-quarter reported no prescription coverage, and nearly half of low-income seniors in some states lacked coverage. Wide coverage differences among states highlight implementation challenges and the need for tailored enrollment strategies. Evidence of Medicaid's highly effective coverage delineates the importance of assuring this group's continued protection under Part D plans. Reports of complex drug regimens, multiple prescribing physicians and pharmacies, nonadherence, and reimportation demonstrate the challenges of integrating seniors' prescription care. We discuss MMA's potential to improve quality and the need to monitor performance.