Black Monday was October 19, 1987, when the Dow Jones Industrial Average lost 22.6%, its largest one-day percentage decline.
The crash affected markets worldwide. Hong Kong fell first in the global sequence, followed by markets in Europe and the United States. The Dow dropped 508 points, a very large numerical move for the time, although percentage comparisons are more meaningful across eras.
Researchers have linked the fall to several factors rather than one proven trigger. Valuations were high, interest rates were rising, and automated portfolio-insurance strategies could intensify selling. International market connections also allowed falling prices in one region to influence another.
The episode did not become a second Great Depression. The U.S. Federal Reserve, led by Alan Greenspan, issued a statement supporting the financial system, and markets eventually recovered. The crash nevertheless changed market regulation, including the later development of circuit breakers designed to pause trading during extreme declines.