Major League Baseball (MLB) is the highest level of professional baseball in the United States and Canada. Specifically, Major League Baseball refers to the organization that operates the National League and the American League by a joint organizational structure that has developed gradually between them since 1901 (the National League having been in existence since 1876). In 2000, the two leagues ceased to be separate legal entities, with the commissioner's office assuming all responsibilities for running MLB. MLB effectively operates as a single league and as such it constitutes one of the major professional sports leagues of the United States and Canada. It is currently composed of 30 teams — twenty-nine in the United States and one in Canada. In conjunction with the International Baseball Federation, MLB also manages the World Baseball Classic.