Partner Leagues are a concept created with the re-organization of the minor leagues into the Professional Development League in 2021. While the bulk of the PDL is comprised of minor league teams affiliated with a major league team, the new system also includes within its orbit four other leagues which receive support from Major League Baseball but are not affiliated with teams.
It is a hybrid category. Three of the leagues are fully professional and operated as independent leagues until 2020: the Atlantic League, the American Association and the Frontier League. The fourth league is an amateur summer collegiate league, the re-formed Appalachian League, which replaces the venerable minor league of the same name that was dissolved as part of the reorganization.