In baseball, to tag up is to retouch or remain on the runner's time-of-pitch base until (after) the ball either lands in fair territory or is first touched by a fielder. A runner must, by rule, tag up only when a batted ball is caught by a fielder (without touching the ground). 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, a fielder with the ball can appeal that the runner did not retouch his time-of-pitch base after the ball was touched by a fielder by touching the runner or the runner's time-of-pitch base before he retouches the base, thus tagging the runner out. If the umpire agrees that the runner did not retouch after the ball was touched by a fielder, the umpire will call the runner out, regardless of what the runner had done after the ball is in play. This is a timing play, not a force play. All runs which score before the appeal will count (except those scored by runners that are called out).

Tag or tagging may refer to: