In baseball, a runner must tag up if a batted ball is caught in flight. To tag up is to retouch the runner's time-of-pitch base after the ball is first touched. After a legal tag up, even if the ball was caught in foul territory, runners are free to attempt to advance. On long fly ball outs, runners can often gain a base; when a runner scores by these means, this is called a sacrifice fly. On short fly balls, runners seldom attempt to advance after tagging up.

After a caught fly ball, if a fielder can touch the runner's time-of-pitch base or the runner with the ball before he tags up, the runner is out. This is considered an appeal play. This often occurs on infield line drives.