The Internal Revenue Service (IRS) Whistleblower Program, which was revised and expanded by Congress in 2006, is an important tax administration tool. Information received from tax whistleblowers – individuals who report to the IRS on violations of tax laws by others – has assisted the IRS in detecting tax compliance issues and in collecting additional tax revenue. Tax whistleblowers who report on the underpayment of taxes by others have helped IRS collect almost $2 billion in additional revenue since 2011, when the first high-dollar claim was paid under the expanded program that pays qualifying whistleblowers a minimum of 15 percent of the collected proceeds. These revenues help reduce the estimated $450 billion tax gap—the difference between taxes owed and those paid on time. This book assesses the Whistleblower Office (WO) claim review process; assesses how the WO determines awards; evaluates how the WO communicates with external stakeholders; and evaluates IRS’s policies and procedures for protecting whistleblowers.