What's the difference between an ITIN and an SSN for filing taxes?

A Social Security Number (SSN) is issued to people authorized to work in the U.S., while an Individual Taxpayer Identification Number (ITIN) is a tax-only ID for people who must file or be claimed on a U.S. return but aren't eligible for an SSN. Both let you file, but they aren't interchangeable: an SSN also tracks Social Security earnings and unlocks more credits, while an ITIN exists purely for tax compliance and doesn't grant work authorization or benefits. Common ITIN users are a nonresident spouse or dependent of a filer. If you later become eligible for an SSN, you must stop using the ITIN and notify the IRS so your records merge. Some credits, like the Earned Income Tax Credit, require an SSN, so the number you file under can affect what you can claim.