Eric Harris and Dylan Klebold

Eric David Harris (April 9, 1981 – April 20, 1999) and Dylan Bennet Klebold (/ˈkliːboʊld/ KLEE-bohld; September 11, 1981 – April 20, 1999) were American high school seniors and a mass murderer duo who perpetrated the massacre at Columbine High School on April 20, 1999, in Columbine, Colorado. Harris and Klebold killed 13 students and one teacher and wounded 23 others. After killing most of their victims in the school library, the two died by suicide. At the time, the attack was the deadliest high school shooting in United States history. The ensuing media frenzy and moral panic turned "Columbine" into a byword for school shootings, and the event into one of the most infamous mass shootings.

Harris and Klebold met while attending middle school and gradually became close. By their junior year of high school, they were described as best friends who were dependent on one another. Early reports characterized them as unpopular and frequent targets of bullying; however, many peers later stated that the two were not near the bottom of the school's social hierarchy, as both had numerous friends and active social lives. Columbine High School was alleged to have an intense "jock culture", in which popular students—primarily athletes—received preferential treatment from faculty and peers. Harris and Klebold often wore black trench coats in their everyday attire; as a result, it was initally believed that they were members of a school clique known as the "Trenchcoat Mafia", a group of students who were said to rebel against the school's norms. This was later found to be unsubstantiated, as Harris and Klebold had little to no affiliation with the group.

According to their writings, Harris and Klebold appear to have begun planning the attack by May 1998, nearly a year beforehand, although each had previously made references to a shooting. Over the following eleven months, they secretly built explosives and amassed an arsenal of weapons, planning to attack in April 1999, just weeks before their graduation. Both left behind journals and video recordings, created individually and together, that foreshadowed the massacre and provided insight into their rationale for the shooting. They intended for this material to be widely viewed and to inspire followers, although much of it has never been released by authorities. The FBI concluded in May 2000 that Harris exhibited psychopathic traits, including narcissism, unconstrained aggression, and a lack of empathy, while Klebold was described as an angry depressive with a vengeful attitude toward individuals he believed had mistreated him. However, since neither Harris nor Klebold had been formally diagnosed with any personality disorders during their lifetimes, these conclusions have been criticized and remain debated.

In the years after the massacre, news outlets and commentators have attributed the attack to various motivating factors, including bullying, mental illness, racism, psychiatric medication, and exposure to violence in media. The exact motive for the attack remains inconclusive. Harris and Klebold have become figures in popular culture, portrayed, alluded to, and depicted in various media. Some perpetrators of subsequent attacks have cited the duo as an influence, a phenomenon commonly referred to as the “Columbine effect”.