Most historians place the beginning of the Running of the Bulls in the fourteenth century.  Bullfights have been taking place in Pamplona since 1385, and running with the bulls through the city became a tradition, even though the king of Navarra, Charles II the Bad, tried to suppress it.

Three major summer events were combined in 1591 in Pamplona: the bullfighting festival, the livestock fair (held on the historically tax-free St. Peter’s day) and the festival of San Fermin (originally in October).  For reasons of convenience and better weather, these three festivals were united and are celebrated every year during the week of July 7th, now known as the "Fiestas de San Fermín."  See "Who was San Fermin?".

Letters from 1591 reveal the presence of men on horses to guide the running bulls through the cobbled stone streets to the bullring.  In 1776 the City Council constructed a mandatory fence along the entire route in an effort to make the increasingly popular tradition more safe.  In 1884 the first permanent bullring was built in Pamplona.  Despite several attempts by city leaders to stop the dangerous bull run, popular sentiment prevailed.

The name encierro was first used in 1856; until then, the event had gone by the name entrada. That year, the Plaza del Castillo bullring was replaced with a new one, erected on the site of the present Teatro Gayarre; this marked the beginning of the current 825 meter route.

When Ernest Hemingway attended the festivals in the 1920’s, he was so impressed that he wrote a novel that launched his career. “The Sun Also Rises” was published in 1926 and has made San Fermin famous around the world.  With international television broadcasts in recent years (and sensational online videos), it has become a truly international festival and one of Spain’s most famous events.  See "Ernest Hemingway and San Fermin".

In modern Sanfermines, with the number of runners increasing considerably every year, an encierro can sometimes involve runners being crushed together in heaps. Heaps formed in 1960 and 1993 in the Calle Estafeta, and it has happened about thirty times in the callejón, the narrow alley leading into the bullring itself. Two deaths occurred, one in 1975 and another in 1977. The history of the encierro has involved fifteen deaths – the first was recorded in 1924, and the latest in 2003. On 10 July 1947 and on 13 July 1980, a single bull killed two runners. The encierro run on 12 July 2004 involved the most gorings ever recorded in a modern-day fiesta. Eight people were gored by bulls bred by the Jandilla estate. A further ten people were injured and treated in hospital.  During the past several years there have been no fatalities directly resulting from the bulls.