If you're traveling to Canada, Mexico, the Caribbean or Bermuda, you'll need a passport to get back into the U.S.
The change in requirements kicks in today after being delayed from last January.
It affects airline passengers returning from the four countries.
The new requirements were temporarily put on hold after a flood of applications and complaints about months-long delays in getting a passport.
The State Department says it has processed 30-million passport applications over the last two fiscal years.