This article describes the income replacement ratio as a measure of retirement income adequacy and identifies several issues analysts must consider when calculating a replacement ratio. The article presents the income replacement ratios experienced by participants in the original sample cohort of the Health and Retirement Study (HRS), who were born between 1931 and 1941. Replacement ratios are shown by the respondent’s birth cohort, age when first classified as retired in the HRS, and preretirement income quartile. Median replacement ratios fall as the retirement period grows longer.