Identity theft tax refund fraud is a persistent, evolving threat to honest taxpayers and tax administration. It occurs when an identity thief files a fraudulent tax return using a legitimate taxpayer's identifying information and claims a refund. GAO was asked to review IRS's efforts to combat IDT refund fraud. This report, the first of a series, examines (1) what IRS knows about the extent of IDT refund fraud and (2) additional actions IRS can take to combat IDT refund fraud using third-party information from, for example, employers and financial institutions. To understand what is known about the extent of IDT refund fraud, GAO reviewed IRS documentation, including the Identity Theft Taxonomy. To identify additional actions IRS can take, GAO assessed IRS and SSA data on the timing of W-2s; and interviewed SSA officials and selected associations representing software companies, return preparers, payroll companies, and others.