PURPOSE As Medicare Part D enters its fifth year, we assess how the supply side of the market has evolved and what research has shown about how Medicare drug coverage has affected consumers. METHODS We conduct descriptive data analyses to explore the varied nature of Medicare standalone prescription drug plans (in terms of both price and non-price features), examine features associated with high enrollment, and show trends over time in both plan design and enrollment patterns from 2006 to 2010. We also review existing evidence about Part D's effects on drug access for beneficiaries and conclude with a discussion of current policy concerns. FINDINGS Medicare Part D has been successful in certain ways, but several areas of concern remain. Although it is a measure of success that 90% of Medicare beneficiaries now have drug coverage, efforts continue to reach the vulnerable populations who are not yet signed up. Use of medications (and relative use of generics) has increased under the program, while out of pocket costs have fallen. Policymakers continue to question government's role in areas such as negotiating prices directly with pharmaceutical manufacturers and limiting the number of plans offered. Results from data analysis indicate, among other things, high growth in premiums, whereas plans have become less generous by certain measures. ORIGINALITY This chapter brings together data on all plans offered in Medicare Part D standalone drug coverage market and shows new evidence on the landscape's rapid evolution.