Instead, a foreign national, whether in the United States as an immigrant, nonimmigrant or unauthorized (illegal) alien, is classified as a resident or nonresident alien for federal tax purposes. In general, an individual is a nonresident alien unless he or she meets the qualifications under either residency test:
Green card test: the individual is a lawful permanent resident of the United States at any time during the current year, or Substantial presence test: the individual is present in the United States for at least 31 days during the current year and at least 183 days during the current year and previous two years. For example, an individual will be treated as a nonresident alien if he or she has a closer connection to a foreign country than to the United States, maintains a tax home in the foreign country, and is in the United States for fewer than 183 days during the year. A residency definition in an income tax treaty will override these residency rules. Instead, the Code treats these individuals in the same manner as other foreign nationals—they are subject to federal taxes and classified for tax purposes as either resident or nonresident aliens. This classification is for tax purposes only and does not affect the individual's immigration status. While most taxpayers file tax returns using their Social Security number (SSN) as an identifier, individuals who are ineligible to receive an SSN file their returns using an individual taxpayer identification number (ITIN). Furthermore, in the 112 th Congress, legislation has been introduced that would impose, with some differences, an SSN requirement for claiming the additional child tax credit, any part of the child tax credit, or any credit or refund (e.g., H.R. 1196 ). Two of these bills— H.R. 3630 and H.R. 5652 —have been passed by the House; however, H.R. 3630 was enacted into law ( P.L. 112-96 ) without the SSN provision. The mechanism of using an SSN requirement for restricting the eligibility of unauthorized aliens to claim tax credits may be imprecise. Taxation of Income

		Resident Aliens

Resident aliens are generally subject to the same federal income tax laws as citizens of the United States. There are limited circumstances in which a nonresident alien's U.S. source income is not subject to U.S. taxation. For example, some interest income that is not connected with a U.S. trade or business (e.g., portfolio interest) is exempt from U.S. tax. With respect to the issue of double coverage and taxation, agreements generally provide that individuals are only covered by the social security program (and therefore only subject to the program's taxes) in the country where they are working, although individuals who are covered in their home country and temporarily assigned by their employer to work in the other country are exempt from coverage in that country. In 2004, the United States signed an agreement with Mexico, which has been controversial. It has not yet been transmitted to Congress. The Consolidated Appropriations Act, 2012 prohibits the Social Security Commissioner or Social Security Administration (SSA) from using any of the funds appropriated by the act to pay compensation to SSA employees to administer Social Security benefit payments under any U.S.-Mexico totalization agreement that would not otherwise be payable. Other legislation has been introduced, the Loophole Elimination and Verification Enforcement Act (or LEAVE Act), that would state it is the sense of the House that the U.S.-Mexico totalization agreement "is inappropriate public policy and should not take effect." Another bill introduced in the 112 th Congress would address a constitutional issue with the existing totalization agreement process.
A question that often arises is whether unauthorized aliens and other foreign nationals working in the United States are subject to U.S. taxes. The federal tax consequences for these individuals are dependent on (a) whether an individual is classified as a resident or nonresident alien and (b) whether a tax treaty or totalization agreement exists between the United States and the individual's home country.
In general, an individual is a resident alien if he or she is a lawful permanent U.S. resident or is in the United States for a substantial period of time during the current and past two years (the "substantial presence" test). Otherwise, he or she will typically be classified as a nonresident alien. Resident aliens are generally taxed in the same manner as U.S. citizens. Nonresident aliens are subject to different treatment, such as generally being taxed only on income from U.S. sources. Exceptions exist for aliens with specific types of visas or employment.
An individual who is in the country unlawfully is, like any other alien, classified as either a resident or nonresident alien. This classification is for tax purposes only, and it does not affect the individual's immigration status. These individuals' eligibility to claim the earned income tax credit is restricted because the tax code requires that taxpayers claiming the credit provide their Social Security number (SSN), as well as those of their spouse and dependents. Unauthorized aliens are ineligible for SSNs, and therefore file their tax returns using an individual taxpayer identification number (ITIN). In the 112th Congress, legislation has been introduced that would, with some differences, impose an SSN requirement for claiming the additional child tax credit (e.g., H.R. 3630, H.R. 5652, H.R. 3275, and H.R. 1956), for claiming any part of the child tax credit (H.R. 3444 and S. 577), or for claiming any credit or refund (e.g., H.R. 1196). Two of these bills—H.R. 3630 and H.R. 5652—have been passed by the House; however, H.R. 3630 was enacted into law (P.L. 112-96) without the provision.
Finally, the provisions of an income tax treaty or totalization agreement may reduce or eliminate taxes owed to the United States. An income tax treaty is a bilateral agreement between the United States and another country that addresses the income tax treatment of each country's residents while in the other country, primarily with the intent of reducing the incidence of double taxation. Totalization agreements are bilateral treaties that address social security taxes. In 2004, the United States signed a totalization agreement with Mexico, but it has not yet been transmitted to Congress for review. In the 112th Congress, the Consolidated Appropriations Act, 2012 prohibits the Social Security Commissioner or Social Security Administration (SSA) from using any of the funds appropriated by the act to pay compensation to SSA employees to administer Social Security benefit payments under any U.S.-Mexico totalization agreement that would not otherwise be payable. Other legislation has been introduced that would state it is the sense of the House that the U.S.-Mexico totalization agreement "is inappropriate public policy and should not take effect" (H.R. 1196), or address a constitutional issue with the manner in which totalization agreements are disapproved by Congress (S. 181).