Location: Chicago
Passenger traffic (March, 2005-March, 2006): 76,510,003
Percent change over year: 1.3

Chicago’s O’Hare has four terminals—three for domestic and one for international travel—which can make for a tough transfer if you’re connecting to a different airline that is not in the same terminal as your first stop. That scenario plays out when flying Northwest Airlines () to American Airlines. O’Hare is the second-busiest passenger airport in the world.