text: about illegal immigrants? what is a w 7 form. basically use form w-7 to apply for an irs individual taxpayer identification number (itin). an is a nine-digit number issued by the u.s. internal revenue service (irs) to individuals who are required for u.s. tax purposes to have a u.s. taxpayer identification number but who do not have, and are not eligible to obtain, a social security number (ssn).  the itin is for federal tax purposes only. it does not entitle you to social security benefits and does not change your immigration status or your right to work in the united states. also, individuals filing tax returns using an itin are not eligible for the earned income credit (eic). ssns. do not complete form w-7 if you have an ssn or you are eligible to obtain an ssn. you are eligible for an ssn if you are a u.s. citizen or if you have been admitted by the united states for permanent residence or u.s. employment.