Testimony issued by the Government Accountability Office with an abstract that begins "Long-term budget simulations by GAO and others show that we face large and growing structural deficits due primarily to known demographic trends and rising health care costs. Reducing the annual tax gap--the difference between what taxpayers timely and accurately pay in taxes and what they should pay under the law--could help the nation cope with these long-term fiscal challenges. The tax gap arises through the underreporting of tax liabilities, underpayment of taxes due or "nonfiling" of required tax returns. This testimony discusses the findings of GAO's recent tax gap report. It addresses the significance of reducing the tax gap, measuring the extent of the tax gap, collecting data on reasons why noncompliance occurs, and the Internal Revenue Service's (IRS) strategies for reducing the tax gap."