Passports are now required if travelling by plane

US-born citizens and U.S.-Permanent Residents
Carry identification to establish your citizenship such as a Birth Certificate and least one ID card with photo. You do NOT require a passport or visa if crossing the border by car or boat. All persons travelling by plane will require a passport to gain re-entry to the United States. Permanent residents of the U.S. should also bring their "Green Card." If you are a naturalized U.S. citizen, you should carry your naturalization certificate. For more information visit www.travel.state.gov

U.S. Temporary Residents
Anyone who carries a Temporary Resident Card, Form 1-688, or Employment Authorization Card, 1-688A or 1-688B) must carry their passport and may also require a visa depending on their country of citizenship.