Date: Oct. 19, 1987
(MARIA BASTONE/AFP/Getty Images)
Decline: 20.5 percent
Conditions: Ronald Reagan's "Morning in America" was in its home stretch and the economy was growing at 4 percent, but federal budget deficits were piling up fast and talk of American decline was in the air. There is no clear reason for the sell-off, which followed a three-day decline of 10 percent. Analysts have blamed everything from program trading run amok to worries about monetary policy. Whatever the cause, within two years the S&P had regained its 1987 peak and resumed the greatest bull market in history.