A Personal Taxpayer Identification Number (ITIN) is a number issued by the Internal Revenue Service (IRS) for tax payment purposes and is assigned to people who are not eligible for a social security number. As of January 1, 2017, all ITINs not used on a federal tax return at least once in the last three years will no longer be valid for use unless renewed. ITINs requested before 2013 must also be renewed. At SOS we help you both in the application and in the renewal.