The Great Depression was a period of severe global economic downturn that occurred from 1929 to 1939. It was characterised by high unemployment rates and widespread business failures around the world. The economic contagion began around September 1929 in the United States, the largest economy in the world. Economic historians usually consider the catalyst of the Great Depression to have been the devastating Wall Street stock market crash of October, a sharp decline in U.S. stock prices often referred to as 'Black Tuesday'.