The decennial census has been conducted in years ending in "0" since 1790, as required by the U.S. Constitution. Article I, Section 2 states that:
"Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers . . . The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct."
Accordingly, a census is taken every 10 years. U.S. marshals conducted the enumeration between 1790 and 1870, and specially trained enumerators carried out the census beginning in 1880. The earliest decennial censuses were conducted under the authority of the Secretary of State. The Department of the Interior assumed responsibility in 1849. Finally, upon its creation in 1902, the Department of Commerce and Labor's permanent U.S. Census Bureau oversaw the census.
The first decennial census was a "simple" count. It consisted of six questions and counted approximately 3.9 million people for purposes of apportioning the U.S. House of Representatives. As the 2010 census approaches, one million enumerators will assist the Census Bureau in counting more than 300 million of the nation's inhabitants. In addition to apportioning state representation, 2010 census data will be used to make decisions effecting legislation and spending on housing, highways, hospitals, schools, assistance programs, and scores of projects and programs that are vital to the health and welfare of the U.S. population and economy.
Additional information, including lists of the questions asked during each census, can be found on this Web site in Through the Decades.
For historical information about the decennial censuses see, Measuring America: The Decennial Censuses From 1790 to 2000 and the procedural histories of each census.