The free baggage allowance per traveler varies by airline and changes frequently. Most airlines are now charging for each bag checked.
There may be exceptions to this policy based on the type of ticket purchased and preferred frequent flyer memberships. Weight restrictions also apply. Upon check-in, a service fee may be charged by the airline for baggage.
Some airlines restrict the number or type of carry-on baggage allowed on board. Please check the airline links below. The Transportation Security Administration continues to restrict certain substances. An update may be found at TSA.
Below are links for the major airlines baggage policies.
(though these links are current, occasionally the airline will modify their websites)