The five domestic levels of MiLB today are, from lowest to highest, Rookie League[a], Single-A, High-A, AA (or Double-A) and AAA (or Triple-A).

This system proved unworkable, however, as there was no way to ensure competitive balance, and financially unsound clubs often folded during the season.

These were loose groups of independent clubs which agreed to play a series of games over the course of one season for a championship pennant.

[5] The NAPBL became involved in the later stages of the negotiations to develop rules for the acquisition of players from their leagues by the NL and the AL.

The NA leagues were still fiercely independent, and the term minor was seldom used in reference to them, save by the major-market sportswriters.

[10] Commissioner Kenesaw Mountain Landis initially resisted Rickey's plan but, ultimately, the Great Depression drove teams to establish systems like Rickey's to ensure a steady supply of players, as many NA and independent teams could not afford to keep their doors open without the patronage of Major League Baseball.

Deprived of this option by the flat-fee policy, minor-league teams had little choice but to sell players as soon as they drew major-league interest.

For example, in 1930, Landis released Fred Bennett to free agency after the St. Louis Browns sent him to play in their farm system for a third consecutive year, then seen as a violation of the player's rights.

This would lead to a legal battle with a court voiding the "secret absolute control" of players in 1931 without eliminating the farm system.

[11] However, later rule changes by the owners would formalize and limits major league clubs' ability to "option" players to the minors.

[13] However, the rise of television in years after led to many minor-league clubs losing supporters for now-watchable major-league teams.

Minor League Baseball's popularity would recover in the 1990s and continue to grow throughout the 2000s, reaching a new peak attendance of 41.6 million in 2009.

[16]: 15  The different levels represented different levels of protection for player contracts and reserve clauses:[16]: 15 After the National Association of Professional Baseball Leagues was founded in 1901, classifications were redefined:[17][18][19] † Draft fee set an amount for a team in a higher class to select a player; n/a for Class A as it would be up to each team to negotiate with an interested major league club.

[22] The PCL would revert to Triple-A in 1958,[24] due to increasing television coverage of major league games and in light of the Dodgers and Giants moving to Los Angeles and San Francisco, respectively.

[16]: 525–527 After the 1962 season, the Triple-A American Association—which had lost key markets such as Milwaukee, Kansas City, Minneapolis–Saint Paul and Houston to the Major Leagues since 1953—disbanded.

Meanwhile, at the Double-A level and below there were even more significant changes: Designations below Class A disappeared because the lower levels could not sustain operation during a large downturn in the financial fortunes of minor league baseball, due to factors including the rise of television broadcasts of major league sports across broad regions of the country.

[34] Manfred rebuked Minor League Baseball for releasing the negotiations to the public and threatened to cut ties with MiLB altogether.

As of the 2022 season, the minor league system is divided into four classes: Triple-A (AAA), Double-A (AA), High-A (A+), and Single-A (A).

[49] Some Triple-A players are "career minor leaguers", former prospects whose skill growth has halted and who are not likely to advance to MLB, unless as a temporary replacement.

[53] Minor leagues with the Rookie classification play a shortened season that traditionally began in mid-June and ended in late August or early September.

[58] From September 1 to the end of the regular season, teams were allowed to expand their active rosters up to 40 players, the size of the major league reserve list.

The director of player development and the general manager usually determine the initial assignments for new draftees, who typically begin playing professionally in June after they have been signed to contracts.

In more modern times, released players often sign with independent baseball clubs, which are scouted heavily by major league organizations.

[61] Many players have signing bonuses and other additional compensation that can run into the millions of dollars, although that is generally reserved for early-round draft picks.

[67] Players on the injured list (IL) can be sent to the minor leagues to aid in rehabilitation following an injury, typically for one or two weeks.

[67] Former Minnesota Twins star Joe Mauer, who missed most of the first two months of the 2011 season due to a difficult recovery from arthroscopic knee surgery after the 2010 season, reported to Minnesota's Class A-Advanced Florida State League team, the Fort Myers Miracle, which is based in their spring training facility in Fort Myers.

This allowed Trout to stay closer to the Angels compared to the team's Triple-A affiliate, the Salt Lake Bees.

The program aimed to recruit more athletic, energetic, and dedicated individuals who would also have high morals and integrity standards.

[76] A strike action occurred at the start of the 2006 season,[77] spurred by a disagreement over salaries and resulting in the use of replacement umpires until an agreement was reached after two months.

[100] Nearly every minor-league team has its own local radio contract, though unlike their major-league counterparts, these generally consist of only one or two individual stations.