In American football, clock management is an important aspect of game strategy. The team who holds the lead in the game will want to use as much time as possible, while the team that is trailing will want to conserve time so that there is enough to try to score.
Upon kickoff the clock is started when a member of the receiving team touches the ball, or, if the member of the receiving team touches the ball in their end zone, carries the ball out of the end zone. The clock is stopped when that player is tackled or goes out of bounds. (The clock never starts if the receiving team downs the ball in their own end zone for a touchback.) The clock is then restarted when the offense snaps the ball for their first play and continues to run unless one the following occurs:
- A player carrying the ball goes out of bounds (within the last five minutes of a half).
- A forward pass is ruled incomplete. (Included in this is the rule whereby the quarterback can "spike" the ball near himself without being called for intentional grounding. The team loses no yardage by doing so, only the down, and gains the benefit of the stopped clock.)
- Either team calls for a time-out or an official calls for a time-out, perhaps because a player is injured or there is a penalty on the play. Officials will restart the clock after an official time-out, but not a team time-out, has concluded unless another of the conditions applies.
- A player from either team scores a touchdown, or a player from the offensive team kicks a field goal.
- Possession of the football is transferred between teams for any reason.
- In college football, the clock is briefly stopped when a team earns a first down to allow the chain crew to reposition themselves. The NFL has no such stoppage.
- The clock runs out. If this happens during a play, the current play continues until the ball is dead.
Each team is given three time-outs per half which they can use to stop the clock from running after a play.