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. This book examines what IRS knows about the extent of IDT refund fraud and additional actions IRS can take to combat IDT refund fraud using third-party information from, for example, employers and financial institutions. The book also assesses the quality of IRS's IDT refund fraud cost estimates, and IRS's progress in developing processes to enhance taxpayer authentication.