Great Depression also known in U.K. as the "Great Slump" was a dramatic, worldwide economic downturn beginning in some countries as early as 1928. The beginning of the Great Depression in the U.S. is associated with the stock market crash, on Oct. 29, 1929 known as "Black Tuesday".
The Depression has devastating effects in both industrialized countries and those which exported raw materials. International trade declined sharply, as did personal income, tax revenues, prices and profits.
The key cause of the Depression was the expansion of the money supply in the 1920's that lead to unsustainable credit driven boom. One reason for the monetary inflation was to help Great britain, which in the 1920's was struggling with it's plan to return to the gold standar at pre-war (WWI) parity. Returning to the gold standard at this rate meant that the british economy was fully deflationary pressure.