In 2006, the United States Centers for Medicare and Medicaid Services implemented Medicare Part D to provide outpatient prescription drug insurance to disabled and older adults. In creating Part D, a key provision to address quality included medication therapy management (MTM) programs designed to increase proper and safe use of medications among targeted Part D beneficiaries. A preponderance of evidence shows that Part D has increased medication affordability and accessibility; however, what remains less clear is whether it has improved the quality of medication use and optimized health outcomes. Now in its sixth year, Part D is undergoing its first major revision with the gradual elimination of the coverage gap by 2020. Therefore, now is a good time to review the accumulated evidence on the impact of Part D and MTM programs on quality of medication use to help inform future policy decisions and research directions. In this review, we find that Part D’s net effect on quality of medication use mainly has been positive. Cost-related medication nonadherence improves moderately and treatment interruptions happen less than expected. However, vulnerable subgroups, such as sicker and dual-eligible beneficiaries, experienced lags in improvement. Beneficiaries who entered the coverage gap consistently experienced interruptions and displayed worsening medication adherence after entering the gap, with generic-only gap drug coverage offering only suggested limited benefit. Such findings can serve as baseline information as the coverage gap phases out. Limited availability of data is the biggest barrier to research in Part D. Part D’s overall effect on health outcomes and adverse medical events, such as hospitalizations, is inconclusive because of inadequate evidence to date. Similarly, no evaluation of quality of medication use is available on utilization management strategies and MTM programs delivered under Part D. Future research will need to further examine the added value of Part D and address whether it optimizes health outcomes in the Medicare population. As the current economic recession increases the pressure to cut costs, the effect of future spending restrictions, such as restrictions on coverage subsidies, will also be of special concern.