A running of the bulls (Spanish: encierro, from the verb encerrar, 'to corral, to enclose'; Occitan: abrivado, literally 'haste, momentum'; Catalan: correbous, 'run-bulls') is an event that involves running in front of a small group of bulls, typically six but sometimes ten or more, that have been let loose on sectioned-off streets in a town, usually as part of a summertime festival. Particular breeds of cattle may be favored, such as the toro bravo in Spain, also often used in post-run bullfighting, and Camargue cattle in Occitan France, which are not fought. Bulls (non-castrated male cattle) are typically used in such events.

The most famous bull-run is the encierro held in Pamplona during the nine-day festival of Sanfermines in honor of Saint Fermin. It has become a major global tourism event, today very different from the traditional, local festival. More traditional summer bull-runs are held in other places such as towns and villages across Spain and Portugal, in some cities in Mexico, and in the Occitan (Camargue) region of southern France. Bull-running was formerly also practiced in rural England, most famously at Stamford until 1837.