Major League Baseball (MLB) is a professional baseball organization, the oldest of the four major professional sports leagues in the United States and Canada. A total of 30 teams play in the National League (NL) and American League (AL), with 15 teams in each league. The NL and AL were formed as separate legal entities in 1876 and 1901 respectively. After cooperating but remaining legally separate entities beginning in 1903, the leagues merged into a single organization led by the Commissioner of Baseball in 2000. The organization also oversees Minor League Baseball, which comprises about 240 teams affiliated with the Major League clubs. With the World Baseball Softball Confederation, MLB manages the international World Baseball Classic tournament.
Baseball's first openly all-professional team was founded in Cincinnati in 1869. (There had been teams in the past that paid some players, and some that had paid all players but under the table.) The first few decades of professional baseball were characterized by rivalries between leagues and by players who often jumped from one team or league to another.
The period before 1920 in baseball was known as the dead-ball era; players rarely hit home runs during this time. Baseball survived a conspiracy to fix the 1919 World Series, which came to be known as the Black Sox Scandal. The sport rose in popularity in the 1920s, and survived potential downturns during the Great Depression and World War II. Shortly after the war, Jackie Robinson broke baseball's color barrier.
The 1950s and 1960s were a time of expansion for the AL and NL, then new stadiums and artificial turf surfaces began to change the game in the 1970s and 1980s. Home runs dominated the game during the 1990s, and media reports began to discuss the use of anabolic steroids among Major League players in the mid-2000s. In 2006, an investigation produced the Mitchell Report, which implicated many players in the use of performance-enhancing substances, including at least one player from each team.
Today, MLB is composed of 30 teams: 29 in the United States and one in Canada. Teams play 162 games each season and five teams in each league advance to a four-round postseason tournament that culminates in the World Series, a best-of-seven championship series between the two league champions that dates to 1903. Baseball broadcasts are aired on television, radio, and the Internet throughout North America and in several other countries throughout the world. MLB has the highest season attendance of any sports league in the world with more than 73 million spectators in 2015.