Eric David Harris (April 9, 1981 – April 20, 1999) and Dylan Bennet Klebold (; September 11, 1981 – April 20, 1999) were an American mass murderer duo who perpetrated the Columbine High School massacre on April 20, 1999. Harris and Klebold killed 13 people and wounded 24 others at Columbine High School, where they were seniors, in Columbine, Colorado. After killing most of their victims in the school's library, they later committed suicide. At the time, it was the deadliest high school shooting in U.S. history. The ensuing media frenzy and moral panic following the massacre led to "Columbine" becoming a byword for school shootings, and becoming one of the most infamous mass shootings ever perpetrated.

Harris and Klebold were both born in 1981. Harris was born in Wichita, Kansas, but moved around frequently as a child due to his father's occupation in the United States Air Force, while Klebold was born and raised near Columbine. Harris' family eventually settled in Colorado in 1992. Shortly after, Harris and Klebold met, while they were in the 7th grade. Over time, they became increasingly close. By the time they were juniors in high school, they were described as inseparable. There are differing reports; some say Harris and Klebold were very unpopular students once they were upperclassmen, and frequent targets of bullying, while others say they were not near the bottom of the school's social hierarchy and each had many friends, along with active social lives. Columbine High School was known to have an intense "jock culture", which saw popular students, mainly athletes, benefit from special treatment from faculty and other students.

According to their journal entries, Harris and Klebold had seemed to begin planning the attack by May 1998, nearly a year before the attack. Throughout the next eleven months, Harris and Klebold meticulously built explosives and gathered an arsenal of weapons. Both Harris and Klebold each left behind several journal writings and home videos, ones they made both alone and together, foreshadowing the massacre and explaining their motives. Harris and Klebold hoped this content would be viewed by the public extensively and inspire followers, although much of the evidence has never been released by authorities.

Harris and Klebold often wore trench coats in school, and in general, as part of their everyday outfits, so after the massacre, it was widely believed Harris and Klebold were part of a clique in school called the "Trenchcoat Mafia", a group of misfits in the school who supposedly rebelled against the popular students. This turned out to be untrue, as neither Harris nor Klebold had any affiliation with the group. The pair's aforementioned writings and videos gave insight into their rationale for the shooting. The FBI concluded that Harris was a psychopath, who exhibited narcissistic traits, unconstrained aggression, and a lack of empathy, while Klebold was concluded to be an angry depressive with a vengeful attitude toward individuals who he believed had mistreated him. However neither Harris nor Klebold were formally diagnosed with any personality disorders prior to the attack, so this conclusion is often debated. In the following years, various media outlets attributed multiple motivating factors to the attack, including bullying, mental illness, racism, psychiatric medication, and violence in music, movies, and video games. Despite these conclusions, the exact motive for the attack remains inconclusive.

Harris and Klebold have become pop culture icons, with the pair often portrayed, referenced and seen in film, television, video games, music and books. Many killers since the shooting have taken inspiration from the pair (dubbed the Columbine effect), either hailing them as heroes, martyrs and gods, or expressing sympathy for them. Harris and Klebold also have a fanbase, who have coined the term "Columbiners", who write fan fiction and draw fan art of them. Others have also dressed as the duo for cosplay or Halloween.