An ITIN, or Individual Taxpayer Identification Number, is a tax processing number only available for certain nonresident and resident aliens, their spouses, and dependents who cannot get a Social Security Number (SSN). It is a 9-digit number, beginning with the number “9”, formatted like an SSN (NNN-NN-NNNN).
An Adoption Taxpayer Identification Number (ATIN) is a temporary nine-digit number issued by the IRS to individuals who are in the process of legally adopting a U.S. citizen or resident child but who cannot get an SSN for that child in time to file their tax return.
Beginning January 1, 2011, if you are a paid tax preparer you must use a valid Preparer Tax Identification Number (PTIN) on returns you prepare. Use of the PTIN no longer is optional. If you do not have a PTIN, you must get one by using the new IRS sign-up system. Even if you have a PTIN but you received it prior to September 28, 2010, you must apply for a new or renewed PTIN by using the new system. If all your authentication information matches, you may be issued the same number. You must have a PTIN if you, for compensation, prepare all or substantially all of any federal tax return or claim for refund.
If you do not want to apply for a PTIN online, use Form W-12, IRS Paid Preparer Tax Identification Number Application (PDF). The paper application will take 4-6 weeks to process.