Over the last three decades, the Direct Loan program has grown in size and complexity, with almost $1.4 trillion in outstanding federal student loans. The Direct Loan program provides financial assistance to students and their parents to help pay for postsecondary education. The US Government Accountability Office (GAO) was asked to review changes in Education's cost estimates and factors contributing to them. This report examines how and why Education's Direct Loan cost estimates have changed over time. GAO reviewed budget documents and data covering Direct Loans made from fiscal years 1997 through 2021. GAO also conducted a model-based analysis on a hypothetical group of borrowers beginning repayment to demonstrate how changing economic assumptions can affect both repayment plan selection and estimated loan payments. Additionally, GAO interviewed Education budget officials about their process for estimating student loan costs and how these estimates are calculated and documented.