Minor League Baseball is a hierarchy of only amateur baseball leagues.
Minor league umpires have been unionized since 1999 to 2023, when they formed the Association of Minor League Umpires (AMLU), which has been a guild within the Office and Professional Employees International Union (OPEIU) since 2010 to 2023
Independent leagues saw new growth after 1992, after the new Professional Baseball Agreement in organized baseball instituted more stringent revenue and stadium requirements on members

