The order applies to everyone traveling to the U.S. on international flights, including U.S. citizens. You will need to show negative test results even if you are flying on a private jet or charter flight. There are exceptions for children under age 2, airline crews and federal law-enforcement agents and members of the military traveling for duty.

What countries are covered?

All of them. Passengers need to show proof of a negative test when traveling to the U.S. from any country, including Mexico and the Caribbean. Airlines can seek temporary waivers for certain countries where testing supplies are inadequate, but carriers haven’t yet said which countries might fall into this category. The requirement doesn’t apply to U.S. territories, such as Puerto Rico.

The universal testing requirement goes into effect Jan. 26. People arriving from the United Kingdom already have been subject to similar testing requirements that went into effect in December, following the emergence of a new coronavirus strain there.

Airline executives have said they expect some short-term hiccups as the new policy goes into effect but say they believe testing will help restore confidence in travel in the long run. United Airlines Holdings Inc., for example, has said it is working with partners to increase the supply of tests in places like Mexico, where the new requirements have put a damper on appetite for trips to beach resorts.

“We are going to work really really hard to make sure it’s really, really easy to travel with United even with the new testing requirements,” Toby Enqvist, United’s chief customer officer, said.

What kind of test will I need, and when? What happens if I don’t have my results?

U.S.-bound air travelers must get tested no more than three days before flying and bring written or electronic proof of the results. Airlines can accept both PCR and rapid antigen tests. The CDC has said home diagnostic kits that are analyzed in a lab should qualify, if the kits have been approved by national health authorities.

If you don’t have the documentation with you, airlines won’t allow you to board, according to the CDC’s order.

Airlines are still working out the exact protocols they will use to check test results. Customers coming from the U.K. on United Airlines show test results in the lobby before security, but the details might not be the same for other countries, a United spokeswoman said.

To streamline the verification process, American Airlines Group Inc. is expanding access to a health passport app where travelers can upload documents such as negative Covid-19 test results. Starting Jan. 23, the VeriFly app will be available for American Airlines customers traveling from all international destinations.

What if I have been vaccinated?

Even if you have been vaccinated for Covid-19, you still will need to show proof of a negative test.

What if I recently had Covid-19 and got better?

If you have tested positive for Covid-19 in the past three months but no longer have symptoms, the CDC doesn’t recommend getting tested again. If you are in this group and have met the criteria to end isolation, the CDC says you can travel as long as you have written permission from a health-care provider or public-health official. Bring your positive test result and the doctor’s letter to show the airline in lieu of a negative test result.

Does this mean the U.S. has lifted bans on most travel from Europe, the U.K. and other countries?

No. Mr. Biden rejected a last-minute effort by former President Donald Trump to lift those travel restrictions on Jan. 26, citing the worsening pandemic and more-contagious variants of the virus emerging around the world. That means people who aren’t U.S. citizens or permanent residents generally can’t come to the U.S. from most of Europe, the U.K., Brazil, China and Iran, with a few exceptions. Likewise, many countries don’t allow travelers to enter from the U.S., or continue to impose lengthy quarantine requirements on arrival.

What if I need to change my travel plans now?

Airlines already have done away with most international change fees and are adding more flexibility for people who need to adjust their plans and get to the U.S. before the new testing requirements go into effect.

American and Delta Air Lines Inc., DAL -1.65% for instance, will allow people to rebook international tickets to the U.S. that had been scheduled through Feb. 9, as long as new travel starts on or before Jan. 25. That means you can rebook to fly before testing is required without paying higher last-minute fares. United is waiving fare differences for travel originally scheduled through Feb. 15.

Do I need to get tested again or quarantine when I get back to the U.S.?

The CDC recommends people get a second test three to five days after travel and stay home for seven days (or 10 days without a second test). Testing on its own isn’t a substitute for social distancing and wearing face masks, CDC officials said.

President Biden has said he believes people arriving from abroad should be required to self-quarantine; his executive order says air travelers will be required to follow the CDC’s guidelines, including self-isolation recommendations, “to the extent feasible.” It isn’t yet clear how that might be enforced, and the order doesn’t specify. The order also directs federal agencies to develop a plan for ensuring compliance with CDC guidelines and to consider other public-health measures, including consideration of additional testing immediately prior to departure.