Medicare Part D was implemented 4 years ago. Despite the fact that public-use Part D data were unavailable until late 2008, researchers have used alternate data to examine the effect of Part D on drug use and out-of-pocket costs. In a systematic review of Medline from 2006 to October 2009, the literature about drug use and costs after implementation and during the transition period and coverage gap was summarized. Studies presenting original results regarding drug use and costs after Part D implementation were included. Case reports and series and simulation studies were excluded. Of 552 originally identified articles, 26 met selection criteria: 13 regarding the overall effect of Part D in the year(s) after implementation, seven describing the Part D transition period, and six concerning the coverage gap. Part D implementation was associated with a 6% to 13% increase in drug use and a 13% to 18% decrease in patient costs. The transition period was associated with no significant changes in use or costs for elderly dual-eligible beneficiaries, but effects in other populations were mixed. Entry into the coverage gap was associated with a 9% to 16% decrease in drug use and increases in costs of up to 89%. In summary, studies examining disparate data regarding the implementation of Part D found consistent positive effects on drug use and costs but revealed unfavorable trends in the coverage gap. The effect of the Part D transition period remains unclear. Although public-use data will validate these results, policymakers can use the existing evidence to inform changes and enhancements to Part D immediately.