Some 14 MLS teams operate clubs in the USL Championship and League One, and four more have affiliates in those leagues.

Major League Soccer and United Soccer League entered the sixth season of their partnership in 2018. The 2018 USL regular season begins on Friday, with 33 teams participating this season.

Fourteen clubs are slated to compete in 2025. While the USL Championship has a number of teams on the verge of competing in Major League Soccer, USL League One is much more of a development league.