An Individual Taxpayer Identification Number, or ITIN for short, is for people who have a federal income tax filing or reporting requirement, but who are not eligible for a Social Security number. Taxpayers who need an ITIN can mail a completed form W-7, along with the required identification documents and federal tax return, to the IRS. Check out the instructions for Form W-7 for a list of acceptable identification documents. Only send original identification documents or a copy that was certified by the agency that issued it. Another option, rather than mailing original documents like your passport, is that you can work with a certifying acceptance agent, known as a CAA, who's authorized by the IRS to verify your documents. You can also make an appointment to get help at selected IRS Taxpayer Assistance Centers. Remember that any ITIN that isn't used on a tax return at least once in three years will expire at the end of the third year. If it does, and you need to file a tax return, be sure to submit form W-7 to renew your expired ITIN. This will help avoid delays in the processing of your tax return and tax refund. You'll also want to make sure any other ITIN listed on your tax return isn't expired – like an ITIN for your spouse or a dependent. For more information on ITINs and where you can get help, go to IRS.gov/itin.