With salary caps that vary based on player classification and roster regulations that feel like those long legal disclaimers that everyone definitely reads, Major League Soccer has developed a unique set of rules for its teams to operate under. Since the league began in 1996, MLS has grown and evolved with every passing year, modifying and adding to some already pretty wordy rulebooks.

While the rules in place have helped the league grow and succeed, they don’t really make for a quick, easy read. That’s why we’re breaking down some of Major League Soccer’s more complicated guidelines, right before they change again ahead of the 2024 season.

Building a team in this league isn’t as simple as signing enough players to field an 11. Clubs need to pay attention to a list of regulations, with special attention given to where players are from and where they played their youth soccer. Player designations are closely tied to compensation, but for now, we’ll just cover what each roster slot can be used for.

Roster Classifications

Every team is comprised of up to 30 players, separated into two distinct classifications: the senior roster and the supplemental roster. All 30 players are eligible for gameday selection, but the kinds of players on each side of the roster differ in several key areas.

The senior roster consists of slots 1-20. While no club is required to fill spots 19 or 20, every team has the option to do so. These tend to be the players that get the most playing time, typically the starting 11 and the usual substitutes that appear on the game-day team sheet.

Roster slots 21–30 are collectively known as the supplemental roster. This category is typically used for depth pieces, including Reserve Minimum Salary Players, Senior Minimum Salary Players, Homegrown Players, Generation adidas Players, and players rostered under a few other, more specific designations. These spots are frequently occupied by either young players at the beginning of their careers, or experienced veterans nearing the end of theirs.

Player Categories

All 30 players on a roster are further classified into player categories, mainly in an effort to ensure that the league is developing local players and directly improving the quality of the home nation’s national team, a common practice in leagues around the world. 

Categories include: 

• Domestic Players: U.S. citizens, permanent residents (i.e., a green card holder) by the Roster Compliance Date, holders of a special status (e.g., have been granted refugee or asylum status) or a player who qualifies under the Homegrown International Rule. There is no limit as to the number of U.S. domestic players on a U.S. club's roster.

• Homegrown: A club may sign a player to a contract without subjecting him to the MLS SuperDraft if the player has been a member of that club’s youth academy for at least one year and has met the necessary training and retention requirements. There is no limit to the number of Homegrown players a club may sign.

• International Players: Any player that doesn’t fit the requirements to be a domestic player falls into the international category.

• Homegrown Internationals: The player became a member of an MLS club academy, either in the U.S. or Canada (or a Canadian-approved youth club), no later than the year in which he turned 15 years old and signed his first professional contract with MLS or an MLS club's affiliate (MLS NEXT Pro).

• Designated Players: The Designated Player Rule allows clubs to acquire up to three players whose total compensation and acquisition costs exceed the maximum salary budget charge, with the club bearing financial responsibility for the amount of compensation above the charge. This rule was put into place prior to the LA Galaxy’s signing of David Beckham in 2007.

• Young DP: A Designated Player who is 23 years old or younger during the league year is classified as a Young Designated Player. 

• U22 Initiative Players: Each MLS team will have up to three U22 Initiative slots that occupy one of the 20 existing Senior Roster slots. The number of U22 Initiative slots available to each team will be based on that team’s use of its third Designated Player slot. A player must be 22 years old or younger in the first year he is eligible to play in an MLS game (e.g., a player is not eligible for 2023 if he turns 23 in 2023). A player who signs at age 22 or younger may continue to occupy a U22 Initiative Slot through the year in which he turns 25, with stipulations related to Homegrown and non-Homegrown status.

• Special Discovery Players: Special Discovery Players must be 27 years old (or younger than the age of 27) during the league year under consideration. There may be no more than one Special Discovery Player per club in the league. A club may have more than one Special Discovery Player on its Senior Roster at any given time if the club receives the additional player(s) via trade.

In 2023, 233 international roster slots were available across the league. This allows each team to have eight international players, but because these spots are tradable on a season-long basis, it’s not uncommon for teams to have more or less. With trades taken into account, there is no maximum number of international players a club can have, but no more than 233 could be in the league at the same time in 2023. That’s roughly 27% of the 870 players that could make up the league if every club used all 30 roster slots.

The rules are the same for the league’s Canadian clubs in Vancouver, Toronto, and Montreal, but they get the added bonus of having both U.S. and Canadian citizens count as domestic players. They must have a minimum of three Canadians on their rosters, though, a requirement that doesn’t apply to U.S.-based clubs. This is just one of several small differences in roster building details between American and Canadian clubs.

This is by no means an exhaustive guide to building an MLS roster, but it’s a good place to start. If you’re ambitious enough to read through the official rules, have at it.