Between 1965 and 2010, most federal student loans were issued by private lending institutions and guaranteed by the government, and most student loan borrowers made fixed monthly payments over a set period--typically 10 years. Since 2010, however, all federal student loans have been issued directly by the federal government, and borrowers have begun repaying a large and growing fraction of those loans through income-driven repayment plans. Introduced as a way to make student loan repayment more manageable, income-driven plans limit payments to a percentage of borrowers' income and allow for loan forgiveness after 20 or 25 years. In this report, the Congressional Budget Office examined how income-driven plans differ from plans that require fixed monthly payments, how enrollment in income-driven plans has changed over time, and how those plans are projected to affect the federal budget. [This report was written with guidance from Sebastien Gay.]