October 19, 1987, is known as Black Monday, the day global stock markets suffered their most severe one-day percentage collapse in modern history.
The Dow Jones Industrial Average fell 22.6 percent in a single session, a larger one-day percentage decline than the famous crash day of 1929. Selling spread rapidly from markets in Asia and Europe to the United States, demonstrating how increasingly connected financial exchanges had become.
Several explanations have been proposed, including high valuations, worries about interest rates and trade, portfolio insurance strategies, and automated or program trading. No single cause fully explains the speed and scale of the fall. Unlike the 1929 crash, Black Monday did not lead to a decade-long depression.
The episode prompted regulators and exchanges to strengthen market safeguards. U.S. exchanges later introduced circuit breakers, which can temporarily halt trading after unusually large declines and give investors time to assess rapidly changing conditions.