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 major leagues, the National League and the American League, by means of a joint organizational structure which has existed between them since 1903. Major League Baseball teams play a 162-game season. The American League operates under the Designated Hitter Rule, but the National League does not (inter-league, all-star and World Series game rules are determined by the home team's league rules). 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 often referred to as "America's Sport" or "America's Game." 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 (currently 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 and out of the Major League by virtue of their performance. Major League Baseball is mostly funded by private enterprises, but also partially funded directly by public taxes . MLB 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 held that baseball is not interstate commerce and therefore not subject to federal antitrust law despite baseball's own references to itself as an "industry" rather than a "sport." This ruling has been weakened only slightly in subsequent years.

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.