Today, approximately 43 million Americans hold a federal student loan. When these borrowers fall behind on payments, they become delinquent on their loans; once the loans reach 270 days past due, borrowers are in default. As of March 2021, roughly 1 in 5 borrowers was in default, according to data from the U.S. Department of Education. Failing to repay a student loan can have serious, long-term financial consequences. Income-driven repayment (IDR) plans, which were first made available to borrowers in 1995, are designed to make federal student loan repayment more affordable. A growing body of research examines how borrowers use these plans and whether such plans effectively meet the needs of those struggling with delinquency and default. This report reviews the goals and structure of income-driven plans and identifies themes in research on borrowers' experiences in repayment to help policymakers better understand the benefits, drawbacks, and potential effectiveness of reforms to income-driven plans. This report also proposes principles for reform that would address these four key problems with the structure and implementation of IDR plans. Finally, this report identifies unresolved questions and research that are needed to help policymakers move forward with reforms to the student loan repayment system.