In general, individuals are eligible for Medicare if they (or their spouse) worked for at least 10 years in Medicare-covered employment and are at least 65 years old and are a citizen or permanent resident of the United States of America.
Individuals who are under 65 years old can also be eligible if they are disabled or have end stage renal disease. People under 65 and disabled must be receiving disability benefits from either Social Security or the Railroad Retirement Board for at least 24 months before Medicare automatic enrollment occurs.
Many beneficiaries are dual-eligible. This means they qualify for both Medicare and Medicaid. In some states for those with certain income, Medicaid will pay the beneficiaries Part B premium for them (most beneficiaries have worked long enough and have no Part A premium), and also pay any drugs that are not covered by Part D.