When should I give a payer a W-8BEN versus a W-9?

You give a W-9 if you're a U.S. person—a citizen, resident alien, or U.S. entity—to certify your taxpayer ID so payers report your income normally. You give a W-8BEN if you're a nonresident alien, to certify foreign status and, where applicable, claim a reduced treaty withholding rate on U.S.-source income like interest, dividends, or royalties. Submitting the wrong one causes problems: a nonresident who files a W-9 may have income misreported, while a U.S. resident who files a W-8BEN may face unnecessary 30% withholding or backup-withholding issues. Your status can change—once you become a resident alien under the green-card or Substantial Presence Test, you should switch your bank and brokerage forms from W-8BEN to W-9. Review the forms on file with your employer and broker after any change in residency status so withholding stays correct.