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. 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