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The sport of baseball has several professional leagues throughout the world; that is, leagues where the players are paid to play.

Major League Baseball in North America consists of the National League and the American League. Historically, teams in one league never played teams in the other until the World Series, in which the champions of the two leagues played against each other; this changed in 1997 with the advent of interleague play.

In addition to the major leagues, many North American cities and towns feature minor league teams. An organization officially styled Minor League Baseball, formerly the National Association of Professional Baseball, oversees nearly all minor league baseball in the United States and Canada. The minor leagues are divided into classes AAA, AA, A, and rookie league. These minor-league teams are affiliated with major league teams, and serve to develop young players and rehabilitate injured major leaguers. The Mexican League is a Minor League Baseball member league that operates without affiliations to major league teams. There are also a number of non-Minor League Baseball leagues that exist independently of the influence of the major leagues. The Northern League and the Can-Am League (formerly the Northeast League) are two of the top independent minor leagues in North America.

During the late 19th and early 20th centuries, African-American players were barred from playing the major leagues, though several did manage to play by claiming to be Cubans or Indians. As a result, a number of parallel Negro Leagues were formed. However, after Jackie Robinson began playing with the major-league Brooklyn Dodgers in 1947, the Negro Leagues gradually faded. The process of integration did not go entirely smoothly; there were some ugly incidents, including pitchers who would try to throw directly at an African-American player's head. Now, however, baseball is fully integrated, and there is little to no racial tension between teammates.

Between 1943 and 1954, the All-American Girls Professional Baseball League fielded teams in several Midwestern towns.

Japan has had professional baseball since the 1930s. Currently, it consists of two leagues, the Central League and the Pacific League, each with six teams.

Professional leagues exist in Korea, Taiwan, the Dominican Republic, and Latin America, the Netherlands, Italy (from 1948), Germany (from 1951), Australia, and the rest of Europe.