The US government says it may reduce flights at 40 large airports. This could start on Friday if leaders do not end the shutdown.
The Federal Aviation Administration, or FAA, plans to cut about 10% of flights in the busiest places.
Because of the shutdown, air traffic controllers are working without pay. Many are tired and worried about money.
Some controllers are taking other jobs or calling in sick. This creates staff shortages and slows the system.
Passengers may see more delays and cancellations. Some airlines say long international flights will be safe, but short domestic trips could change.
If your flight is canceled, you can ask for a refund, even on a non-refundable ticket. It may help to book a backup flight.