The Columbine High School massacre was a school shooting and attempted bombing that occurred on April 20, 1999, at Columbine High School in Columbine, Colorado, United States. The perpetrators, twelfth grade (senior) students Eric Harris and Dylan Klebold, murdered 12 students and one teacher. Ten students were killed in the school library, where the pair subsequently committed suicide. Twenty-one additional people were injured by gunshots, and gunfire was also exchanged with the police. Another three people were injured trying to escape. At the time, it was the deadliest high school shooting in U.S. history. The shooting has inspired dozens of copycat killings, including many deadlier shootings across the world. The word "Columbine" has become a byword for school shootings.

In addition to the shootings, Harris and Klebold planted several homemade bombs in the school, although they failed to detonate. Two bombs were set up as diversions at another location away from the school, one of which (partially) detonated. The motive remains unclear, but they had planned for around a year and hoped to massacre the most victims in U.S. history, which at the time meant exceeding the death toll of the Oklahoma City bombing.

The police were slow to enter the school and were heavily criticized for not intervening during the shooting. The incident resulted in the introduction of the Immediate Action Rapid Deployment tactic, which is used in active shooter situations. Columbine also resulted in an increased emphasis on school security with zero tolerance policies. Debates and moral panic were sparked over guns and gun control laws, high school cliques, subcultures (e. g. goths), outcasts, and school bullying, as well as teenage use of pharmaceutical antidepressants, the Internet and violence in video games and movies.

Many impromptu memorials were created after the massacre, including victims Rachel Scott's car and John Tomlin's truck. Fifteen crosses for the victims and shooters were also erected on top of a hill in Clement Park. The crosses for Harris and Klebold were removed later following controversy. The Columbine Memorial began planning as a permanent memorial in June 1999 and opened to the public on September 21, 2007.