The Columbine High School massacre occurred on April 20, 1999, when twelfth-grade students Eric Harris and Dylan Klebold murdered twelve students and one teacher at Columbine High School in Columbine, Colorado. The shooting was planned as a terrorist attack that would cause "the most deaths in U.S. history," but the motive has never been ascertained. The shooting was a unique set of circumstances, the magnitude of which no one had dealt with before. The county sheriffs department knew Harris and Klebold had been making pipe bombs and threatening their schoolmates, but the warrant to search Harriss house was never executed. The shooting inspired over 80 copycat attacks, and its impact keeps on growing.

There are many misconceptions about what happened at Columbine. For example, Harris and Klebold were not members of the Trenchcoat Mafia, did not listen to Manson, were not bullied, harbored no specific grudges against any one group, and did not "snap" because of some last-straw traumatic event. They ordered everybody to get up, said how long they had been waiting for this, and seemed to be enjoying themselves, shouting things like "Woo!" after shooting. The shooting lasted for several hours, and it would be nearly three hours before the gunmen were confirmed dead.

There are many different perspectives on what really happened at Columbine, and it is a complex and tragic event that has had a lasting impact on American society.