The Social Security Administration (SSA) receives reports of earnings for the U.S. working population each year. Earnings data are used to administer the Social Security programs and to conduct research on the populations served by those programs. The administrative needs of SSA and other agencies have changed over time and, as a result, there have been numerous changes to the main source of SSA's earnings data, which is known as the Master Earnings File (MEF). By documenting the history, content, limitations, complexities, and uses of the MEF (and data files derived from the MEF), this article serves as a resource for researchers who use earnings data to study work patterns and their implications. It is also a resource for policymakers and administrators who must understand the data used in administering current-law programs and the data available to inform potential changes to those programs.