The worldwide stock-market collapse that began in October 1987 was commonly called Black Monday.
On October 19, 1987, the Dow Jones Industrial Average fell 508 points, or 22.6%, its largest one-day percentage decline. Markets in other countries also suffered severe losses, making the event a global crash rather than an isolated U.S. sell-off. The date became known as Black Monday, although some countries experienced their largest falls on different calendar days because of time-zone differences.
Several explanations have been proposed, including high valuations, concerns about economic conditions, international currency tensions, and automated portfolio-insurance trading. No single cause fully explains the worldwide speed and scale of the decline. Regulators later introduced or strengthened circuit breakers and other measures intended to slow extreme price movements. The crash was severe, but it did not produce a depression comparable to the 1930s Great Depression.