A player leaves the field and a teammate from the bench takes the place. That exchange is a substitution. It allows coaches to manage fatigue, tactics, and injury.

Rules limit the number and timing of substitutions in many sports. Some codes allow unlimited rolling substitutions; others restrict them to a fixed number of permanent changes. Substitutions can refresh pressing intensity, introduce specialist skills, or protect a lead with defensive reinforcements.

Players must be ready to contribute immediately after entering. Coaches who time substitutions well often gain a decisive edge in the closing stages. The substitution board or official signal formalizes the change so that everyone knows who is eligible to participate.

• Replacement of one player with another during a contest
• Used for tactical, fitness, or injury reasons
• Regulated by sport-specific rules
• Can alter the balance of a match