Major League Baseball today would be unrecognizable without the large number of Latin American players and managers filling its ranks. Their strong influence on the sport can trace its beginnings to professional leagues established south of the border and in the Caribbean nations in the 1940s. This narrative history of Latin American baseball leagues during the 1940s and 1950s provides an in-depth, year-by-year chronicle of seasonal leagues in the seven primary baseball-playing areas in the region: Mexico, Nicaragua, Panama, Venezuela, Cuba, the Dominican Republic, and Puerto Rico. The success of these leagues, and their often acrimonious competition with U.S. Organized Baseball, eventually ushered in a new era of contract concessions from owners and general labor advancements for players that forever changed the game.