In 2006, the US 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) programmes 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 programmes on the quality of medication use to help inform future policy decisions and research directions. In this review, we found that Part D's net effect on quality of medication use has mainly been positive. Cost-related medication nonadherence improved moderately and there were fewer than expected treatment interruptions. 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 limited benefit. Such findings can serve as baseline information as the coverage gap phases out. Limited availability of data is the greatest barrier to research into 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 with respect to utilization management strategies and MTM programmes delivered under Part D. Future research will need to further examine the added value of Part D and address whether Part D 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.