Black Monday is the name commonly given to the global, sudden, severe, and largely unexpected stock market crash on October 19, 1987. In Australia and New Zealand, the day is also referred to as Black Tuesday because of the time zone difference from other English-speaking countries. All of the twenty-three major world markets experienced a sharp decline in October 1987. When measured in United States dollars, eight markets declined by 20 to 29%, three by 30 to 39% (Malaysia, Mexico and New Zealand), and three by more than 40% (Hong Kong, Australia and Singapore). The least affected was Austria (a fall of 11.4%) while the most affected was Hong Kong with a drop of 45.8%.

Out of twenty-three major industrial countries, nineteen had a decline greater than 20%. Worldwide losses were estimated at US$1.71 trillion. The severity of the crash sparked fears of extended economic instability or even a reprise of the Great Depression.

On Black Monday, the DJIA fell 508 points (22.6%), accompanied by crashes in the futures exchanges and options markets. This was the largest one-day percentage drop in the history of the DJIA. Significant selling created steep price declines throughout the day, particularly during the last 90 minutes of trading.

Deluged with sell orders, many stocks on the NYSE faced trading halts and delays. Of the 2,257 NYSE-listed stocks, there were 195 trading delays and halts during the day. Total trading volume was so large that the computer and communications systems in place at the time were overwhelmed, leaving orders unfilled for an hour or more. Large funds transfers were delayed for hours and the Fedwire and NYSE SuperDot systems shut down for extended periods, further compounding traders' confusion.