If air travel is in your plans for you and Pooch, you will need to purchase an airline crate. Airlines require these unless your dog is small enough to be considered carry-on luggage (under 10 pounds).
Pets are considered Live Cargo, and are often shipped in the cargo hold of an airplane unless that specific plane has a separate, climate-controlled area.

Air Circulation
Airline kennels must have sufficient air circulation. Be sure that the kennel has airways all along the top of the crate walls for proper ventilation, and a mesh doorway. Without proper ventilation your pet could expire from heat or suffocate due to lack of oxygen.

Properly Latching Door
A door that does not latch closed properly could cause all kinds of problems both in and out of the plane. If the door swings open, you could end up with a dog running loose in the cargo hold, or worse, around on the runway. Add to the hazards of a severely stressed animal on the loose and somebody could be bitten.

Airline Approval
Not all crates are airline approved! Check for the sticker that states the crate has air approval before you buy.

Proper Size
An airline will not ship an animal in a crate that is too small. Crates that are too big are fine, but you will pay more in fees and there may be a very smelly mess to clean afterwards. Stressed pets may defecate in the crate, especially if there is plenty of room to do so. Proper sizing also sure that in colder climates, your pet will be able to retain it's body heat better. The crate should be just big enough for your dog to stand up without touching the ceiling, turn around comfortably, and lay down comfortably.

Mesh hole size on the Door
The holes in the wire mesh of the door must not be big enough for your dog to stick his nose through. This prevents him from possibly biting any airline employees during loading and unloading.