Reporting for pet travel is less than perfect, however, and the government collects no numbers on how many pets travel each month. Airlines are required to report each time there is a problem.
Pets who fly in the main cabin -- such as cats -- must have their own reservations. (It's also a chance for the airline to collect its $100, one-way fee for bringing a pet into the cabin.) American only allows up to seven pets per flight (a maximum of five in coach) on most flights, but limits that number to two pets on regional jet flights run by American Eagle.
Carry-on pets are not allowed to or from Hawaii or on trips across the oceans.
Checked pets travel in kennels in a hold under the plane, just like checked luggage, though they are placed in a special pressurized and temperature-controlled area. But that doesn't necessarily mean it is comfortable. In winter, airlines may require documentation certifying that your pet is acclimated to temperatures lower than 45 degrees.
Regulations about carrier size and, of course, fees, vary by airline, but American for example, charges $150. Unlike carry-on pets, those that are checked don't require reservations on American, but the airline warns that sometimes capacity is reached and pets are accepted on a first-come, first-served basis. That warning is mostly for dog show participants, but you never know when such a show is in town.
Delta limits its flights to only four pets in the main cabin and charges $125. Checking a pet on Delta will cost you $200 each way.