How the Minor Leagues Are Organized: Triple-A to the Complex Leagues
The levels of the affiliated minor leagues, from Triple-A down to the complex leagues and the Dominican Summer League, who controls the players, how affiliation works, and how to read a prospect's stat line.
Every major-league club sits on top of a pyramid. Below the 26 players on the active roster are the men on the 40-man roster who are in the minors, and below them are several hundred more players spread across four full-season levels, a set of complex leagues and a summer league in the Dominican Republic. The system exists for one purpose: to turn players who sign or are drafted at 17 to 22 into the few who can play in the majors.
Since the 2021 season the system has had a clear structure. Each major-league club has four full-season affiliates, at Triple-A, Double-A, High-A and Single-A, plus complex-league and Dominican Summer League teams where the youngest and rawest players begin. The structure was simplified in a restructuring of the minor leagues that year, and it is much easier to follow than the older system of rookie leagues, short-season leagues and several classes of A ball.
This article takes the system level by level, then explains who runs it, who pays the players, how affiliation agreements work, how players move up and out, and how to read a prospect's numbers without being fooled by the level, the league or the ballpark. The league names below are those in use as of the 2025 season; for current alignments, check MLB's official site at MLB.com.
The ladder, from the top
| Level | Leagues | What it is for |
|---|---|---|
| Triple-A | International League, Pacific Coast League | The last stop before the majors; prospects ready to be called up, veterans on the fringe, and major leaguers on rehabilitation assignments |
| Double-A | Eastern League, Southern League, Texas League | Often the level where a prospect's skills are tested against advanced pitching and hitting |
| High-A | Midwest League, South Atlantic League, Northwest League | A first full-season test for many college draftees and the better young players |
| Single-A | California League, Carolina League, Florida State League | The entry level of full-season ball for many players |
| Complex leagues | Florida Complex League, Arizona Complex League | Young draftees and international signees in their first years; also injury rehabilitation |
| Dominican Summer League | Dominican Summer League | International signees, many of them 16 to 18 years old, playing in the Dominican Republic |
Each club therefore has four teams that play in a full-season schedule, from April to September, and a few that play shorter schedules in the summer. A club's four affiliates are listed from top to bottom by level, and a promotion moves a player to the next team up.
The levels are not defined by age. They are defined by skill, though age correlates: the players in Triple-A are on average the oldest in the minors, and those in the complex leagues are the youngest. A 19-year-old at Double-A is a prospect worth noting for that reason alone, because he is playing against hitters or pitchers who are several years older.
What changed in 2021
Before 2021 the minor leagues were run by a separate organization, Minor League Baseball, which licensed teams and held agreements with the major-league clubs. The classification system had three levels of A ball, two rookie classes, short-season leagues and a larger number of clubs. In 2021 Major League Baseball took over the governance of affiliated baseball, and the structure was reorganized. There were 160 minor-league affiliates before, and the new structure has 120 full-season clubs, 30 for each of the four full-season levels.
The change eliminated the short-season leagues and the lower rookie classes, moved some teams up and down in level and removed the major-league affiliation for others. Leagues were renamed for 2021 and reverted to their traditional names in 2022. The full account is in the article on the 2021 minor-league restructuring. What matters for this article is that the pyramid now has a consistent shape: one affiliate at each full-season level per club.
Who controls the players
A minor-league team is typically a separate business from the major-league club it affiliates with, although some are owned by the parent club. The division of responsibility is standard. The major-league club signs the players, pays their salaries, provides the coaches and trainers, sets their assignments and controls their promotion and release. The local team owner runs the ballpark, sells tickets and concessions and handles the community side of the business.
This explains how a prospect can be moved between teams in the same organization at a moment's notice. The player does not belong to the Double-A club in a city; he belongs to the major-league organization, which can send him up or down without asking the local owner. It also explains why a minor-league team's results are often secondary to development. The parent club may have a player pitch only three innings, or sit a hitter to protect him, because the aim is to develop the player rather than win a game.
In 2022 minor-league players joined the Major League Baseball Players Association, and they later reached their first collective bargaining agreement with MLB. Pay and working conditions in the minors have been a long-running issue, and the details have changed in recent years; this article does not quote figures, and the current terms are best found in MLB and union announcements.
Affiliation agreements
A minor-league club is affiliated with a major-league club under a written agreement. Under the structure introduced in 2021 these are called Player Development Licenses, and they replaced the earlier Player Development Contracts. The licenses run for a fixed term of years, and geography and the quality of the facilities are part of the arrangement. A major-league club likes its affiliates to be close enough that a call-up can reach the majors quickly, and many clubs place Triple-A affiliates within a day's travel of the home park, though that is not always possible.
Affiliation can change. A club may move an affiliate when an agreement ends or the parties agree to a change, which is why a city's minor-league team can switch parent clubs. The team usually keeps its name and ballpark when this happens, and only the affiliation changes.
Outside the affiliated structure sit independent leagues, such as the Atlantic League and the Frontier League, which are not tied to major-league clubs. Some of these have partnerships with MLB, and the Atlantic League has been used as a testing ground for rules before they were considered for the majors. Their players are not part of a club's farm system, although a major-league club can sign one.
The complex leagues and the Dominican Summer League
The bottom of the pyramid is the least visible. The complex leagues, the Florida Complex League and the Arizona Complex League, are played mostly at clubs' spring-training facilities in those states, often with small crowds or none. Newly drafted high-school players, young international signees who have come to the United States, and players recovering from injuries play there. The article on how teams use the complex leagues explains their role in more detail, including rehabilitation assignments for major leaguers.
The Dominican Summer League is the starting point for many international signees. Players from the Dominican Republic and other countries sign professional contracts at 16 and older and begin in the Dominican Summer League, where clubs have academies. Many do not reach the United States for a year or two, and some never do. How those contracts are signed is covered in the article on how international prospects sign with MLB clubs.
How players enter and move
Players come into the system in two main ways. Most American, Canadian and Puerto Rican players enter through the annual draft, covered in the MLB Draft guide, which assigns each selected player to a club that then places him at a level suited to his age and skill. The order of the first picks is set by the draft lottery, which is open to clubs that did not make the playoffs. Players from other countries mostly enter as international free agents.
Movement after that follows a few rules. A player on a minor-league roster who is not on the 40-man roster can be promoted to a higher minor-league level at any time. A player on the 40-man roster can be optioned to the minors, which allows the club to send him down without losing him, subject to the rules on how many times and for how long. A player can be added to the 40-man roster and called up to the major-league club when the club needs him. Players not on the 40-man roster after several seasons can become minor-league free agents, and an unprotected player who has been in the system for some time can be taken by another club in the Rule 5 draft.
Promotion is not automatic. A player who dominates a level is usually moved up, and one who struggles may repeat a level. Development staffs also use the levels to teach particular skills: a pitcher might be asked to work on a new pitch at Single-A even if he could succeed without it, and a hitter might focus on one part of the strike zone. The article on winter leagues describes how the offseason is used for similar work.
A short history of the farm system
The modern idea of a farm system, in which a major-league club owns or controls a chain of minor-league teams, is usually credited to Branch Rickey, who built one for the St. Louis Cardinals in the 1920s. Before that, minor-league clubs often sold players to the majors one at a time, and a player who developed well could be sold at a high price. Rickey's approach was to control the supply. The idea spread across the majors, and the Cardinals' success in the following decades helped persuade other clubs to follow. The club's history, including the stars its system produced, is in the article on the best players in St. Louis Cardinals history.
In the decades since, the relationship between the majors and the minors has been regulated by agreements between the leagues and then by contracts between clubs. The 2021 reorganization is the latest and most thorough change, moving control of affiliated baseball to MLB itself.
The minors as a laboratory
Because the minors are under MLB's control, they have been used as a testing ground for rules. Examples include the pitch clock, larger bases, limits on defensive shifts, the automatic runner in extra innings and automated ball-and-strike systems, all of which were used in minor-league games before some were adopted in the majors. This is one reason a fan watching a Triple-A game may see rules that differ from those in the majors. For current rules in the majors, see the official rulebook rather than assuming the minors match.
Three typical paths through the system
No two careers move at the same pace, but three paths are common enough to be useful as examples. They are illustrations, not rules, and clubs make their own decisions.
A high-school player drafted at 18 usually begins in a complex league, where he gets his first professional instruction and plays a short schedule. If he develops, he may reach Single-A the next year or the year after, then climb one level a season, which puts a major-league arrival several years away.
A college player drafted at 21 or 22 is older and more finished, so a club may start him at a higher level, perhaps Single-A or High-A, and move him faster. Polished college hitters and pitchers can reach Double-A within their first full year, though skill, not the draft, decides how long they stay.
An international signee from the Dominican Republic or another country often begins in the Dominican Summer League at 16 or 17, then moves to a complex league in the United States and from there up the ladder. Because he starts younger than most draftees, his path to the majors can take longer, and his age relative to each level is a key part of how his progress is judged.
In all three cases the same principle applies: the club tries to match the level to the player's skills and to keep him learning, which sometimes means leaving a good player at a level longer than the box score suggests, or moving a promising one up before he has finished the work at the lower level.
Reading a prospect's stat line
A batting average or earned run average from the minors does not carry over to the majors in a simple way, and three factors explain why.
- Level. A .300 average at Double-A is more impressive than a .300 average at Single-A, because the pitchers are better. A hitter who is younger than the average player at his level is also more promising than one who is older.
- League and park. Some minor-league parks, such as those at high altitude in the Pacific Coast League, are known as hitters' parks, and their statistics need adjusting. The scoring environments of leagues differ as well.
- Role and sample. A pitcher in the minors may be limited to a few innings, and a small number of games can mislead.
For this reason scouts and analysts talk about a player's age relative to his level, his skills, and his underlying numbers such as strikeouts and walks, not just his headline average. The site's minors section lists minor-league results, and the Minor League topic page collects news about prospects.
Common questions
How many levels are there? Four full-season levels, Triple-A, Double-A, High-A and Single-A, plus the complex leagues and the Dominican Summer League.
How many minor-league teams does each MLB club have? Four full-season affiliates, plus complex-league and Dominican Summer League teams.
Do minor leaguers belong to the minor-league team? No. They are under contract to the major-league organization, which controls their assignments.
What is the difference between being optioned and being released? An optioned player stays under contract to the club and can be recalled. A released player is no longer in the organization.
Is the draft the only way in? No. International players sign as free agents, and some players are signed after being drafted by another club or playing in independent leagues.
Where to go next
Read the 2021 minor-league restructuring for how the current pyramid was built, how the MLB Draft works for how most players enter it, and how teams use the complex leagues for the bottom rungs. Browse the site's minor-league pages for scores and standings, and the draft and prospects topic page for the next class of players.