The Medicare Prescription Drug, Improvement, and Modernization Act of 2003 established a new voluntary prescription drug benefit under a new Medicare Part D, effective January 1, 2006. Prescription drug coverage is provided through private prescription drug plans (PDPs) or Medicare Advantage prescription drug (MA-PD) plans. Beneficiaries must enrol in one of these private plans in order to obtain their drug benefits. The program relies on these private plans to provide coverage and to bear some of the financial risk for drug costs; federal subsidies covering the bulk of the risk are provided to encourage participation. At a minimum, plans offer standard coverage or alternative coverage with actuarially equivalent benefits. They may also offer enhanced benefits. All plans are required to meet certain minimum requirements, including those related to beneficiary protections. However, there are significant differences among plans in terms of benefit design, drugs included on plan formularies (i.e., list of covered drugs), cost-sharing applicable for particular drugs, and monthly premiums. In general, beneficiaries can enrol in a plan, or change plan enrolment, when they first become eligible for Medicare or during the annual open enrolment period. The open enrolment period for 2008 is from November 15, 2007, to December 31, 2007. Plans can change from year to year. Beneficiary needs may also change. Therefore, beneficiaries should review their plan choice annually to make sure that their chosen plan continues to meet their needs.