The Running of the Bulls ,from the verb encerrar, to corral, to enclose) is a practice that involves running in front of a small group of cattle, typically six, of the toro bravo breed that have been let loose on a course of a sectioned-off subset of a town's streets.
The most famous running of the bulls is that of the eight-day festival of Sanfermines in honour of Saint Fermin in Pamplona, although they are also traditionally held in other places such as towns and villages across Spain, Portugal, in some cities in Mexico, and southern France during the summer.
According to the Mayor of Pamplona in his foreword to the book Fiesta: How To Survive The Bulls Of Pamplona the origin of this event comes from the need to transport the bulls from the fields outside the city, where they were bred, to the bullring, where they would be killed in the evening. (Although in most cities today the bulls are bred in other parts of the country before being transported to corrals on the outskirts of the city by truck.) During this 'run', youngsters would jump among them to show off their bravado. In Pamplona and other places, the six bulls in the event are still those that will feature in the afternoon bullfight of the same day.
Spanish tradition says the true origin of the run began in northeastern Spain during the early 14th century. While transporting cattle in order to sell them at the market, men would try to speed the process by hurrying their cattle using tactics of fear and excitement. After years of this practice, the transportation and hurrying began to turn into a competition, as young adults would attempt to race in front of the bulls and make it safely to their pens without being overtaken. When the popularity of this practice increased and was noticed more and more by the expanding population of Spanish cities, a tradition was created and stands to this day.