Airline approved pet carriers are a must for anyone flying with their companion animal. While each airline's particular policies may vary somewhat, most U.S. carriers adhere closely to the U.S. Department of Agriculture (USDA) regulations regarding the size of the pet carrier.

When choosing a carrier, make sure that is the proper size for your pet. The pet should have enough room to stand, turn, and lie down. Airline approved pet carriers must have a latch that closes securely and cannot require any special tools to open or close. Most airlines do not allow more than one adult pet or more than two puppies or kittens (younger than 6 months and weighing less than 20 pounds each) in a pet carrier.

Always ensure that your pet carrier is constructed of sturdy material so that your pet will not be able to escape. It should be properly ventilated so that your pet receives plenty of fresh air. Do not place a lock on your pet carrier, as airline personnel need to be able to remove your pet in the event of an emergency.

Few, if any, airlines allow pet carriers with wheels; this is for the safety of animals placed in cargo areas. If your carrier has wheels you will need to determine whether they can be removed before you bring the carrier to the airport. If not, you will likely need to purchase a new carrier.

Check with your specific airline about any size restrictions, especially if you have a very large pet. Some airlines that operate small planes on particular routes will not be able to accommodate very large carriers on all routes. If your pet's carrier is too large to travel on the same plane on which you are flying, the airline may re-route the pet on routes with appropriate equipment. Be sure you ask your airline whether or not your pet will be on the same flights that you will be taking.

Some pets may need to spend a little time getting used to their pet carriers, but many take to them readily. Be patient while training your pet and remember that the time you spend training early will continue to pay off, for you and your pet, his whole life. Begin getting your pet used to his carrier well in advance of your planned flight. If your pet is reluctant to spend time in his carrier, do not force him into it. Leave the door open, allowing him to explore at his own pace. Consider placing treats in the carrier occasionally, to encourage him to go inside.

While specific regulations can vary by company, most airline approved pet carriers meet the standards established by the USDA or the International Airline Transportation Association (IATA).