Little is known about the degree to which individuals are uncertain about their future Social Security benefits, how this varies within the U.S. population, and whether this uncertainty influences financial decisions related to retirement planning. To illuminatethese issues, we present empirical evidence from the Health and Retirement Study Internet Survey and document systematic variation in respondents’ uncertainty about their future Social Security benefits by individual characteristics. We find that respondents with higher levels of uncertainty about future benefits hold a smaller share oftheir wealth in stocks.