In association football, a substitute is a player who is brought on to the pitch during a match in exchange for an existing player. Substitutions are generally made to replace a player who has become tired or injured, or who is performing poorly, or for tactical reasons. A player who has been substituted during a match takes no further part in the game, in games played under the standard International Football Association Board Laws of the Game.

Association football, commonly known as football, or soccer, is a team sport played between two teams of 11 players each, who primarily use their feet to propel a ball around a rectangular field called a pitch. The objective of the game is to score more goals than the opposing team by moving the ball beyond the goal line into a rectangular-framed goal defended by the opposing team. Traditionally, the game has been played over two 45-minute halves, for a total match time of 90 minutes. With an estimated 250 million players active in over 200 countries and territories, it is the world's most popular sport.