Major League Baseball (MLB) is the highest level of play in North American professional baseball. More specifically, Major League Baseball refers to the organization that operates North American professional baseball's two leagues, the National League and the American League, by means of a joint organizational structure which has existed between them since 1903. The American League operates under the Designated Hitter Rule, but the National League does not ('inter-league' game rules are determined by the home team's league).

In 2000, the American and National leagues were officially disbanded as separate legal entities with all rights and functions consolidated in the commissioner's office. MLB effectively operates as a single league and as such it constitutes one of the major professional sports leagues of North America.

Major League Baseball is controlled by an agreement that has undergone several incarnations since 1876, then called the NL Constitution, with the most recent revisions being made in 2005. Major League Baseball, under the direction of its Commissioner, Bud Selig, hires and maintains the sport's umpiring crews, and negotiates marketing, labor, and television contracts. As is the case for most North American sports leagues, the 'closed shop' aspect of MLB effectively prevents the yearly promotion and relegation of teams into the Major League by virtue of their performance.