Crime dropped sharply and unexpectedly in the United States in the 1990s. I conclude that four factors collectively explain the entire drop in crime: increases in the number of police, increases in the size of the prison population, the waning of the crack epidemic, and the legalization of abortion in the 1970s. Other common explanations for declining crime appear far less important. The factors identified are much less successful in explaining fluctuations in crime in the preceding two decades. The real puzzle is not why crime fell in the 1990s, but rather, why crime did not begin falling earlier.