The US economy is officially in recession, says the National Bureau of Economic Research.

Traditionally, a recession has been defined as two consecutive quarters of declining gross domestic product, or the amount of goods and services the country makes. But the Cambridge organization also evaluates personal income, employment, industrial production, and manufacturing and wholesale sales in identifying the economy's peaks and troughs.

Prior to the current downturn, the US has experienced 21 recessions since 1900. Take a look at these past periods of decline.