Major League Baseball (MLB) is the highest level of play in professional baseball. More specifically, Major League Baseball refers to the organization that operates North American professional baseball's two major 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' games are based on the home team). 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 governed 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.

MLB as well maintains a unique, controlling relationship over the sport, including most aspects of minor league baseball. This is due in large part to a 1922 U.S. Supreme Court ruling in Federal Baseball Club v. National League which declared baseball is not considered interstate commerce (and therefore not subject to federal antitrust law), despite baseball's own references to itself as an "industry" rather than a "sport."

The production/multimedia wing of MLB is New York-based MLB Advanced Media , which oversees MLB.com and all 30 of the individual teams' websites. Its charter states that MLB Advanced Media holds editorial independence from the League itself, but it is indeed under the same ownership group and revenue-sharing plan. MLB Productions is a similarly-structured wing of the league, focusing on video and traditional broadcast media.