Recent and projected large federal budget deficits and the need for revenue offsets under the Pay-As-You-Go Act (PAYGO) have generated congressional interest in the feasibility of increasing revenue by reducing the tax gap. The Internal Revenue Service (IRS) defines the gross tax gap as the difference between the tax liability imposed by law for a given tax year and the amount of tax that taxpayers pay voluntarily and on time for that year. It defines the net tax gap as the amount of the gross tax gap that remains unpaid after all enforced and other late payments are made for the tax year. For tax (calendar) year 2006 (the most recent year for which data are available), the IRS estimated a gross tax gap of $450 billion, equal to a noncompliance rate of 16.9%. For the same tax year, IRS enforcement activities, coupled with other late payments, recovered about $65 billion of the gross tax gap, resulting in an estimated net tax gap of $385 billion. The estimated gross tax gap of $450 billion consisted of underreporting of tax liability ($376 billion), nonfiling of tax returns ($28 billion), and underpayment of taxes ($46 billion). (Taxes on illegal activities are excluded from these estimates.) Most of the underreporting of tax liability concerned underreporting of individual income liability ($235 billion). The percentage of individual income that was underreported varied significantly depending on the degree of information reporting and whether or not withholding was required. For the 2006 tax gap estimate, the IRS primarily utilized data from the National Research Program (NRP), which seeks to obtain the optimal balance among research quality, efficiency, and the reduction of taxpayer burden. Estimates of the gross tax gap have been heavily publicized; perhaps as a result, some public officials have emphasized better enforcement of tax laws in order to raise revenue. Three factors affect the dollar amount that can be collected by increased enforcement: some types of unreported income are difficult to detect, some of the detected tax liability cannot be easily collected, and many detected tax liabilities are small relative to enforcement costs. From FY2001 to FY2011, enforcement revenues collected by the IRS increased from $33.8 billion to $55.2 billion. Also from FY2001 to FY2011, IRS staffing for key enforcement occupations rose from 20,203 to 22,184. Over the past four years, the IRS has focused on six strategy priorities: technology modernization, tax return preparers, data analytics, taxpayer service, offshore tax evasion, and workforce job satisfaction. The pursuit of most of these priorities reduces the tax gap. The IRS has put in place the major facets of its tax return preparer initiative. In the 112th Congress, legislation concerning tax compliance has been introduced in the following seven areas: repeal of the 1099 reporting requirement (H.R. 4); identity theft (S. 1534, S. 3432, H.R. 3215, H.R. 3482, and H.R. 6205); free file (S. 1796 and H.R. 2569); contracting (H.R. 829); insurance companies (S. 1693 and H.R. 3157); tax havens (S. 1346, H.R. 2669, S. 2075, and H.R. 3338); and tax avoidance by expatriates (S. 3205).