In 2005, Americans will pay about $2.1 trillion in combined federal taxes, including income, payroll, and excise taxes, or about 16.8 percent of Gross Domestic Product (GDP). However, the amount of taxes paid does not reflect the total cost to taxpayers of the federal tax system. In addition to taxes paid, taxpayers also bear compliance costs and efficiency costs. Understanding the magnitude of these additional costs is important because every dollar spent on compliance and lost due to inefficiency represents a dollar that society could have spent for other purposes. In response to a congressional request for information on the magnitude of the compliance and efficiency costs of the current federal tax system, this study describes the nature of these costs, presents the difficulties associated with estimating them, and summarizes existing estimates of their magnitude. GAO did not make independent estimates of compliance or efficiency costs nor did we replicate any of the studies. GAO is not making any recommendations in this report.