An “ITIN” is an Individual Taxpayer Identification Number assigned by the IRS, for tax administration purposes, to those who are not entitled to a Social Security Number. In this paper, we identify the origins of the ITIN program and describe its implementation by the IRS. We also discuss the role that the ITIN fulfills in tax administration, along with some recent changes in the ITIN laws as part of the Protecting Americans from Tax Hikes and Tax Cuts and Jobs Acts. This history and set of observations should serve as a starting point for future analysis and research into ITINs.