The nation's first census, taken in 1790 when George Washington was president, tallied America's population at more than 3.9 million (compared with more than 281 million in 2000).
The once-a-decade count is required by the U.S. Constitution (Article I, Section 2) to determine the number of seats each state will have in the U.S. House of Representatives.
The government also uses the results to draw congressional and legislative districts, and to distribute more than $400 billion to states and communities each year for everything from roads and schools to health care centers.
By law, the U.S. Census Bureau cannot share an individual's responses with anyone, including other federal agencies and law enforcement entities.
Census information is kept confidential for 72 years. The 1940 census results become available for research at the National Archives beginning April 2, 2012.
The Census Bureau headquarters is in Suitland, Md. (pop. 33,515).
The 2010 census is the 23rd head count in U.S. history.
With only 10 questions, this year's census form is one of the shortest in history.
The most expensive part of the process is when workers must visit households that do not mail back census forms. If 100 percent of households returned their forms, taxpayers would save $1.5 billion.
By law, the Census Bureau must deliver the 2010 population counts to the president by Dec. 31.