In 1901, the National Association of Professional Baseball Leagues, known later Minor League Baseball was formed. Representatives of different minor leagues met at the Leland Hotel in Chicago with the purpose to collaborate while maintaining the independence of the leagues involved.
Several leagues did not sign the agreement and continued to work independently. Fourteen leagues and 96 clubs were members during the first season in 1902. As the NAPBL continued to grow, many considered players like Buddy Ryan, Earl Rapp, and Ike Boone to be as good as those on major league teams. Leagues in the NA would not be truly called minor until Branch Rickey developed the first modern farm system in the 1930s.
In 1946, with the minor leagues poised for unprecedented growth, the higher-level classifications were changed. Class AAA (otherwise known as Triple-A) was created and the three Double-A leagues, the Pacific Coast League, the International League, and the the American Association were reclassified into Triple-A.
The minor league system continued to evolve until 1963 when leagues were organized into one of six classes: Triple-A (AAA), Double-A (AA), Class A-Advanced (High A or A+), Class A (Low A), Class A Short Season, and Rookie. Rookie League teams were further defined into different segments.
In 2019, Major League Baseball proposed dramatic changes to MiLB that would take effect after expiration of the Professional Baseball Agreement at the end of the 2020 season. This included the elimination of many minor league teams. Minor League Baseball released a statement, asserting that it was "unnecessary and unacceptable to wipe out one-quarter of minor league teams" and characterized the proposal as a way "to improve the profitability of MLB".
Minor League Baseball, originally governed through a centralized office was restructured in 2021, with Major League Baseball now handling "all issues related to governance, scheduling, umpiring, license compliance, and other league administration functions.
Throughout the growth of Minor Leagues, many leagues remained independent and unaffiliated with Major League teams. Independent leagues continued in the United States and Canada outside of the purview of organized Minor League Baseball and the Commissioner of Baseball. Independent baseball has become prominent again since 1993.