thumb|Dorothea Lange's Migrant Mother depicts destitute pea pickers in California, centering on Florence Owens Thompson, age 32, a mother of seven children, in Nipomo, California, March 1936. The overall course of the Depression in the United States, as reflected in per-capita GDP (average income per person) shown in constant year 2000 dollars, plus some of the key events of the period. The Depression in international perspective. 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.Great Depression, Encyclopaedia Britannica It was the largest and most important economic depression in the 20th century, and is used in the 21st century as an example of 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 depression had devastating effects in virtually every country, rich or poor. International trade plunged by half to two-thirds, as did personal income, tax revenue, 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 roughly 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. However, 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 Great Depression was triggered by a sudden, total collapse in the stock market. 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 summer of 1930.