The National Basketball Association (NBA) is North America's professional men's basketball league, composed of thirty teams: twenty-nine in the United States and one in Canada. It is an active member of USA Basketball (USAB), which is recognized by the International Basketball Federation as the National Governing Body (NGB) for basketball in the United States. The NBA is one of the four major North American professional sports leagues, which include Major League Baseball (MLB), the National Football League (NFL), and the National Hockey League (NHL).

The league was founded in New York City on June 6, 1946 as the Basketball Association of America (BAA). The league adopted the name National Basketball Association in 1949 after merging with the rival National Basketball League (NBL). The league's several international as well as individual team offices are directed out of its head offices located in the Olympic Tower at 645 Fifth Avenue in New York City. NBA Entertainment and NBA TV studios are directed out of offices located in Secaucus, New Jersey.

1940s and 1950s: The early years

main: Basketball Association of America The Basketball Association of America was founded in 1946 by the owners of the major ice hockey arenas in the Northeastern and Midwestern United States. On November 1, 1946, in Toronto, Ontario, Canada, the Toronto Huskies hosted the New York Knickerbockers, which the NBA now regards as the first game played in the league's history. Although there had been earlier attempts at professional basketball leagues, including the American Basketball League and the NBL, the BAA was the first league to attempt to play primarily in large arenas in major cities. During its early years, the quality of play in the BAA was not significantly better than in competing leagues or among leading independent clubs such as the Harlem Globetrotters. For instance, the 1948 ABL finalist Baltimore Bullets moved to the BAA and won that league's 1948 title, and the 1948 NBL champion Minneapolis Lakers won the 1949 BAA title.

On August 3, 1949, the BAA agreed to merge with the NBL, creating the new National Basketball Association. The new league had seventeen franchises located in a mix of large and small cities, as well as large arenas and smaller gymnasiums and armories. In 1950, the NBA consolidated to eleven franchises, a process that continued until 1954, when the league reached its smallest size of eight franchises, all of which are still in the league (the Knicks, Celtics, Warriors, Lakers, Royals/Kings, Pistons, Hawks, and Nationals/76ers).

The process of contraction saw the league's smaller-city franchises move to larger cities. The Hawks shifted from "Tri-Cities" (the area now known as the Quad Cities) to Milwaukee (in 1951) and then to St. Louis (in 1955); the Royals from Rochester to Cincinnati (in 1957); and the Pistons from Fort Wayne to Detroit (in 1957). In 1960, the Lakers relocated to Los Angeles, and the Warriors moved to San Francisco, in 1963. The following year, the Nationals left upstate New York to bring basketball back to Philadelphia, changing their nickname from "Nationals" to "76ers." This means out of the original eight franchises, only the Knicks and Celtics have not relocated at any point.