An Individual Taxpayer Identification Number (ITIN) 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, besides mailing original documents like your passport, is to work with a certifying acceptance agent (known as a CAA) who’s authorized by the IRS to verify your documents.

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 refund. You’ll also want to be sure any other ITIN listed on your tax return (like an ITIN for your spouse or dependent) isn’t expired.

For more information on ITINs, go to IRS.gov/ITIN.