Current and historic leagues of American football

• The National Football League began in 1920 as the American Professional Football Association.
• The "All America Football Conference" competed from 1946 through 1949. After the 1949 season, three of its teams were admitted into the NFL:
  • Cleveland Browns
  • Baltimore Colts (drop out after 1950, a new Baltimore Colts team started in 1953)
  • San Francisco 49ers
• The "American Football League" competed from 1960 through 1969. All ten teams entered the NFL in 1970 as part of a 1966 merger agreement. There were four different American Football Leagues over the years; this one, the fourth and the only one to last more than two years, changed the nature of professional football in America.
• The "World Football League" competed in 1974 and part of 1975.
• The "United States Football League" competed from 1983 through 1985.
• The "Arena Football League", which plays an indoor version of American Football, has been competing since 1987.
• The "World League of American Football" competed semi-independently for a couple of years in the 1990s, before being transformed into NFL Europe, whose teams act as farm clubs for those in the NFL proper.
• The "XFL" competed in 2001.