Minor leagues associated with Major League Baseball are classified by level of play. Currently, the most experienced and skilled minor leaguers play in the 'AAA'-level ("Triple A") leagues, the International League and the Pacific Coast League. Major League players often move to AAA affiliates during a slump caused by injury or a change in the major league team's lineup. For this reason, AAA is often thought of as the Major 'minor league' and its teams are usually in the biggest cities in the U.S. and Canada not served by Major League Baseball.

Below the 'AAA'-level leagues are the 'AA'-level leagues, the Eastern League, the Southern League, and the Texas League. There are more Single-A leagues, such as the Carolina League, which play a full season, as well as short-season single-A leagues and rookie development leagues.