Two students went on a murderous rampage against their fellow students and staff. 13 were killed and many more injured.
Joining us on Up Close is Frank DeAngelis, who was the principal of Columbine at the time.
He has written a new book that is not just the story of a gun massacre, but also a story of strength, coping and recovery.
Also this week, the chiefs of New York City's three big public library systems make the case together that the budget cuts Mayor Bill de Blasio is proposing will hurt the libraries and New Yorkers.
Up Close: Library budget cuts
And we'll discuss the release of the Robert Mueller report with political consultant Hank Sheinkopf and ABC News political director Rick Klein.
Up Close: Mueller report released