The Great Depression was a worldwide economic downturn starting in most places in 1929 and ending at different times in the 1930s or early 1940s for different countries. It was the largest and most important economic depression in modern history, and is used in the 21st century as a benchmark on how far the world's economy can fall. The Great Depression originated in the United States; historians most often use as a starting date the stock market crash on October 29, 1929, known as Black Tuesday. The end of the depression in the U.S. is associated with the onset of the war economy of World War II, beginning around 1939.

The depression had devastating effects both in the developed and developing world. International trade was deeply affected, as were personal incomes, tax revenues, prices, and profits. Cities all around the world were hit hard, especially those dependent on heavy industry. Construction was virtually halted in many countries. Farming and rural areas suffered as crop prices fell by 40 to 60 percent. Facing plummeting demand with few alternate sources of jobs, areas dependent on primary sector industries such as farming, mining and logging suffered the most. Even shortly after the Wall Street Crash of 1929, optimism persisted. John D. Rockefeller said that "These are days when many are discouraged. In the 93 years of my life, depressions have come and gone. Prosperity has always returned and will again."

The Great Depression ended at different times in different countries; for subsequent history see Home front during World War II. The majority of countries set up relief programs, and most underwent some sort of political upheaval, pushing them to the left or right. In some states, the desperate citizens turned toward nationalist demagogues - the most infamous being Adolf Hitler - setting the stage for World War II in 1939.

The snowball spiral

The Great Depression was not a sudden total collapse. The stock market turned upward in early 1930, returning to early 1929 levels by April, though still almost 30 percent below the peak of September 1929. Together, government and business actually spent more in the first half of 1930 than in the corresponding period of the previous year. But consumers, many of whom had suffered severe losses in the stock market the previous year, cut back their expenditures by ten percent, and a severe drought ravaged the agricultural heartland of the USA beginning in the northern summer of 1930.

In early 1930, credit was ample and available at low rates, but people were reluctant to add new debt by borrowing. By May 1930, auto sales had declined to below the levels of 1928. Prices in general began to decline, but wages held steady in 1930, then began to drop in 1931. Conditions were worst in farming areas, where commodity prices plunged, and in mining and logging areas, where unemployment was high and there were few other jobs. The decline in the American economy was the factor that pulled down most other countries at first, then internal weaknesses or strengths in each country made conditions worse or better. Frantic attempts to shore up the economies of individual nations through protectionist policies, such as the 1930 U.S. Smoot-Hawley Tariff Act and retaliatory tariffs in other countries, exacerbated the collapse in global trade. By late in 1930, a steady decline set in which reached bottom by March 1933.