High Quality Content by WIKIPEDIA articles! Major League Baseball is the highest level of play in North American professional baseball, and is the organization that operates the National League and the American League. In 2000, the two leagues were officially disbanded as separate legal entities, and all of their rights and functions were consolidated in the commissioner`s office. Since that time, Major League Baseball has operated as a single league, and constitutes one of the major professional sports leagues of the United States. It is composed of 30 teams.