Airline alliances are formed when different airlines agree to substantially cooperate with each other. Advantages for customers include more destinations and more convenient connections, while the airlines benefit by feeding passengers to each other instead of to competitors.

For travelers, the most obvious effect of airline alliances is codesharing whereby one airline will sell a seat on a flight with its own flight number, but the flight is performed by a different airline. (Note that not all airlines are part of an airline alliance, and not all airlines within the same alliance do codeshares with each other.)