Since 2006, Medicare beneficiaries have had access to prescription drug coverage offered by private plans, either stand-alone prescription drug plans (PDPs) or Medicare Advantage prescription drug plans (MA-PD plans). In 2013, more than 35 million Medicare beneficiaries are enrolled in Medicare drug plans, including 22.5 million in PDPs and 12.8 million in MA-PD plans; about 11 million Part D enrollees are receiving extra help through the Part D Low-Income Subsidy (LIS) program to pay their drug plan premiums and cost sharing. Part D has evolved since its inception in 2006 due to changes in the private plan marketplace and the regulations that govern the program. This report presents findings from an analysis of the Medicare Part D marketplace in 2013 and changes in drug coverage and costs since 2006.