Homegrown Player Rule in UEFA competitions was introduced in 2006–07 season, with fully in force in 2008–09 season. On top of a maximum 25 players for the List A, clubs had to designate a minimum 8 players that were trained by clubs from the same national league, with 4 of them are from the club own youth system. The rule in turn capped a maximum of 17 foreigners for the club in UEFA competitions. However, such as Lionel Messi who arrived Spain at age 14, some foreigners may counted as domestically trained players and vice versa if the players spent their youth career abroad.
The rule was transpose domestically to English Premier League since 2010.
It was reported that due to the rule, Chelsea F.C. may forced to fill reserve players in their squad for both domestic and European competitions.