Nowadays, most airlines charge a fee for checked bags. The most common fee structure for checked bags for domestic flights in the U.S., as of April 2010, is $25 for the first bag, $35 for the second bag and $100 and up for the third and any additional bags. Keep in mind that these fees are assessed each way; double the fee for round-trip flights. Most U.S. airlines do not charge a fee for the first checked bag on an international flight, and charge $50 for the second checked bag. An exception to this fee structure is Southwest Airlines; they do not charge a fee for the first two checked bags (as of April 2010). All of the above information applies to bags that weigh 50 pounds or less; if a bag weighs more than 50 pounds, additional fees usually apply.
Most airlines charge a fee for overweight bags. Generally, a bag is considered "overweight" by most airlines if the bag exceeds 50 pounds. Depending on the airline, you can expect to pay between $50 and $100 per bag if the bag weighs between 50 and 75 pounds (as of April 2010). If the bag weighs more than 75 pounds, many airlines will not accept it, while others will charge a higher fee. This fee is assessed each way. Double the charges for round-trip flights. The fee may also increase on international flights, depending on your destination. It is helpful to weigh your bag at home before you leave for the airport. If your bag weighs more than 50 pounds, consider unpacking some items or transferring some items to a different bag.
Most airlines charge a fee for checked bags that are considered "oversize" bags. Generally, a bag is oversize if it is larger than 62 linear inches, which is the sum of the bag length, width and height. Airlines charge between $50 and $150 for oversize bags as of April 2010. Remember that the fee is assessed each way, so expect to pay it twice if you are traveling round-trip.
Many items are not permitted in carry-on bags (bags you hand carry onto the plane), but are permitted in checked bags. For example, you cannot bring these items in carry-on bags, but you CAN bring them in checked bags (as of April 2010): liquids in containers larger than 3 ounces; box cutters, scissors, knives or sharp objects; sports equipment such as baseball bats or golf clubs; tools; guns, firearms and ammunition. Note that if you plan to bring guns or ammunition in your checked bag, you should contact the airline before you pack to ask their specific packing guidelines, and bring your permit with you. Expect your bags to be searched thoroughly at the airport.
The following items are prohibited in both checked and carry-on luggage: flammable items such as gasoline or aerosol; explosive materials such as fireworks or dynamite; fire extinguishers; chemicals, like pool chlorine or liquid bleach; lighters; and spray paint.
For a complete list of prohibited items, visit the Transportation Security Administration website.