Non-citizens who live or work in the United States often find their tax treatment confusing. Taxpayers with Individual Taxpayer Identification Numbers (ITINs) may face multiple challenges in fulfilling their requirement to pay taxes or in receiving the tax benefits for which they may be eligible, such as the Other Dependent Credit. This assessment will briefly examine how the tax system treats non-citizens, discuss ITIN filers in the broader landscape of non-citizens, including undocumented workers, and detail the amount of taxes these individuals pay and the tax credits they receive. This study will explore the taxes paid and credits received by ITIN filers over the past several years as well as the barriers individuals face when applying for and using ITINs and related difficulties as they navigate the IRS’s ITIN process from year to year. It will estimate the amount of taxes not paid by individuals eligible for an ITIN who choose to avoid the complicated IRS ITIN guidance. The study will build upon previous TAS work on the IRS administration of the ITIN program, including two prior National Taxpayer Advocate Reports to Congress discussing how the IRS ITIN program was one of the most serious problems facing taxpayers.8 TAS may also consult other experts who have studied taxpayer usage of ITINs. The study will describe the economic characteristics of typical ITIN filers, the challenges they face in meeting their filing obligations, and the deterrent effects these challenges may have on others who are eligible for or who might otherwise obtain ITINs.