Booing is an act of showing displeasure for someone or something, generally an entertainer, by loudly yelling boo! (and holding the "oo" sound) or making other noises of disparagement, such as hissing. People may make hand signs at the entertainer, such as the thumbs down sign. If spectators particularly dislike the performance they may also accompany booing by throwing objects (traditionally rotten fruit) onstage, though the objects may not be meant to physically hurt the performer.

Booing performers has a very long history, The first written record comes from ancient Greece. At the annual Festival of Dionysia in Athens, playwrights competed to determine whose tragedy was the best. When the democratic reformer Cleisthenes came to power in the 6th century B.C., audience participation came to be regarded as a civic duty. The audience applauded to show its approval and shouted and whistled to show displeasure. In ancient Rome, jeering was common at the gladiatorial games, where audience participation often determined whether a competitor lived or died.

In sports, booing by fans is quite common. They may boo particularly-hated players on the opposing team, or any opposing player when there is an intense rivalry between the teams. Unsportsmanlike behavior is also booed, such as intentionally hitting home team batters in baseball or diving in association football or basketball (where it is a technical foul). Booing of referees or umpires after an unpopular ruling is also common.