Airlines are companies that furnish air transport services for freight, passengers or chartered flights. Airlines own or lease the aircraft that are used to carry out one or more of the aforementioned services. An airline may also form an coalition or partnership with another airline or airlines to benefit mutually.

The demand and pricing for air travel services depends on a number of factors, including leisure passenger needs, business passenger needs, demand for business cargo shipments and all of these are of course influenced by overall economic activity of a given area or region.

Overall, the demand for air travel services has risen quite consistently. While annual growth rates during the 80s and 90s ranged between 5-6%, there was a drastic 15% in earlier days of aviation during the 1950s and 1960s.

Growth rates are certainly not uniform across the board and differ from region to region. In areas where deregulation provided greater pricing independence and, in turn, competition, the results were lower fares and sometimes very dramatic spikes in overall growth.

After World War I the U.S. was inundated with airmen. Many of these aviators opted to use their war surplus planes to perform various barnstorming programs for passengers and spectators.

In 1918 the United States Postal Service began using airplanes to experiment with air mail service. They used aircraft services acquired from the United States Army. After the Army flew many air mail missions, the Post Office decided it was time to start their own air mail network, as the Army proved to be undependable.

Eventually, the 1920s brought the advent of passenger airlines. Many of these companies offering passenger service still dealt primarily with transporting mail. Then in the year 1925, Ford bought the Stout Aircraft Company as well as started construction on an all-metal aircraft that became the first American passenger airliner.

Pan American World Airways was the first American airline to go international, and was the only U.S. airline to do so before the 1940s.

Immediately after Word War II there was a surplus of military aircraft, and some designs such as the Douglas C-3 that were used for cargo and troop transport, were converted to civilian passenger service.

Even during the depression the American airline industry was profitable for most airlines and continued to be so until the start of World War II. At this time the U.S. saw much better airline profits than did war-torn Europe. Around this time the airline industry really took off with advances in technology as well as manufacturing procedures of aircraft.