The United States Census is a decennial census mandated by the United States Constitution. The population is enumerated every 10 years and the results are used to allocate Congressional seats (congressional apportionment), electoral votes, and government program funding. Some states or local jurisdictions also conduct local censuses.
The census is performed by the United States Census Bureau. The first census after the American Revolution was taken in 1790 under Secretary of State Thomas Jefferson; there have been 21 federal censuses since that time. The last national census was held in 2000, and the next census is scheduled for 2010.
Decennial U.S. Census figures are based on actual counts of persons dwelling in U.S. residential structures. They include citizens, non-citizen legal residents, non-citizen long-term visitors, and illegal immigrants. In recent censuses, estimates of uncounted housed, homeless, and migratory persons have been added to the directly reported figures.
For years between the decennial censuses, the Census Bureau issues estimates made using surveys and statistical models.
The practice of including non-citizens in the official census figures is controversial as the census is used for the apportionment of seats in the House of Representatives, and derived from that, of electors to the Electoral College. The Census also employs the practice of using hot deck imputation to assign data to housing units where occupation status is unknown. This practice has effects across many areas but is seen by some as controversial because it may increase representation for reliably Democratic districts. However, the practice was ruled constitutional by the U.S. Supreme Court in Utah v. Evans. Groups like the Prison Policy Initiative assert that the census practice of counting prisoners as residents of prisons, not their pre-incarceration addresses, leads to misleading information about racial demographics and population numbers.
The census records and data specific to individual respondents are not available to the public until 72 years after a given census was taken, but aggregate statistical data derived from the census are released as soon as they are available. Every census up to and including 1930 is currently available to the public and can be viewed on microfilm released by the National Archives and Records Administration, the official keeper of archived federal census records. Complete online census records can be accessed for no cost from National Archives facilities and many libraries  , and a growing portion of the census is freely available from non-commercial online sources.  
Data for research purposes are available for all surviving census records from Integrated Public Use Microdata Series (IPUMS). Scanned copies of each of the decennial census questionnaires distributed in the United States from 1960 forward are also available on-line from IPUMS International.
Censuses had been taken prior to the Constitution's ratification; in the early 1600s, a census was taken in Virginia, and people were counted in nearly all of the British colonies that became the United States.
Through the years, the country's needs and interests became more complex. This meant that statistics were needed to help people understand what was happening and have a basis for planning. The content of the decennial census changed accordingly. In 1810 the first inquiry on manufactures, quantity and value of products occurred; in 1840 inquiries on fisheries were added; and in 1850, the census included inquiries on social issues, such as taxation, churches, pauperism, and crime. The censuses also spread geographically, to new states and territories added to the Union, as well as to other areas under U.S. sovereignty or jurisdiction. There were so many more inquiries of all kinds in the census of 1880 that almost a full decade was needed to publish all the results. In response to this, the census was mechanised in 1890, with tabulating machines made by Herman Hollerith. This reduced the processing time to two and a half years .
For the first six censuses (1790-1840) enumerators recorded only the names of the heads of household and a general demographic accounting of the remaining members of the household. Beginning in 1850, all members of the household were named on the census. The first slave schedules were also completed in 1850, with the second (and last) in 1860. Censuses of the late 19th century also included agricultural and industrial schedules to gauge the productivity of the nation's economy. Mortality schedules (taken between 1850 and 1880) captured a snapshot of life spans and causes of death throughout the country.
The first nine censuses (1790-1870) were not managed by the Executive Branch, but by the Judicial Branch. The United States federal court districts assigned U.S. marshals, who hired assistant marshals to conduct the actual enumeration.
See main article: United States Census, 1790. The first Census was taken August 2, 1790. The federal census records for the first census are missing for five states: Delaware, Georgia, Kentucky, New Jersey and Virginia. They were destroyed some time between the time of the census-taking and 1830. The census estimated the population of the United States at 3,929,214.
See main article: United States Census, 1840. The sixth Census was taken June 1, 1840. The census estimated the population of the United States at 17,100,000. The results were tabulated by 28 clerks in the Bureau of the Census.
See main article: United States Census, 1850. The seventh Census was taken June 1, 1850. The 1850 census was a landmark year in American census-taking. It was the first year in which the census bureau attempted to count every member of every household, including women, children and slaves. Accordingly, the first slave schedules were produced in 1850. Prior to 1850, census records had only recorded the name of the head of the household and broad statistical accounting of other household members, (three children under age five, one woman between the age of 35 and 40, etc.).
This was the first census where the American Indians officially were counted, but only those who had 'renounced tribal rules'. The figure for the nation was 40,000.
This was the first census that permitted women to be enumerators.
See main article: United States Census, 1890. The eleventh Census was taken June 2, 1890 because June 1 was a Sunday. The 1890 census announced that the frontier region of the United States no longer existed and therefore the tracking of westward migration would no longer be tabulated in the census. This trend prompted Frederick Jackson Turner to develop his milestone Frontier Thesis.
The 1890 census was the first to be compiled on a tabulating machine, developed by Herman Hollerith. This introduction of technology reduced the time taken to tabulate the census from seven years for the 1880 census to two and a half years for the 1890 census. The total population of 62,622,250 was announced after only six weeks of processing. Ironically, the public reaction to this tabulation was disbelief, as it was widely believed that the "right answer" was at least 75,000,000.
The logistical difficulties in compiling the census drove computing technology for the next fifty years until computers became widespread in industry. IBM's first electronic computer was created primarily to deal with the needs of the census in addition to military and academic uses.
This census is also notable for the fact it is one of only three for which the original data are no longer available. Almost all the population schedules were destroyed as the result of a fire in 1921.
This was the first census that recorded a population exceeding 100 million.
See main article: United States Census, 1940. The sixteenth Census was taken on April 1, 1940. Because of a 72-year privacy law, this census will not be available for public inspection until April 1, 2012.
See main article: United States Census, 1950. The seventeenth Census was taken on April 1, 1950. Because of a 72-year privacy law, this census will not be available for public inspection until April 1, 2022.
See main article: United States Census, 1960. The eighteenth Census was taken on April 1, 1960. Because of a 72-year privacy law, this census will not be available for public inspection until April 1, 2032.
See main article: United States Census, 1970. The nineteenth Census was taken on April 1, 1970. Because of a 72-year privacy law, this census will not be available for public inspection until April 1, 2042.
See main article: United States Census, 1980. The twentieth Census was taken on April 1, 1980. Because of a 72-year privacy law, this census will not be available for public inspection until April 1, 2052.
See main article: United States Census, 1990. The 21st Census was taken on April 1, 1990. Because of a 72-year privacy law, this census will not be available for public inspection until April 1, 2062.
See main article: United States Census, 2000. The 22nd Census took place on April 1, 2000. Because of a 72-year privacy law, this census will not be available for public inspection until April 1, 2072.
See main article: United States Census, 2010. The 23rd Census is planned to take place on April 1, 2010. Because of a 72-year privacy law, this census will not be available for public inspection until April 1, 2082.
The sole purpose of the censuses and surveys is to secure general statistical information. Replies are obtained from individuals and establishments only to enable the compilation of such general statistics. The confidentiality of these replies is very important. By law, no one - neither the census takers nor any other Census Bureau employee - is permitted to reveal identifiable information about any person, household, or business.
Without such protections, those living illegally in the United States or hiding from the government would be deterred from submitting census data.
As with any large collection of personal data which can be traced back to individuals, the potential for abuse of census data exists. During the period 1939 to 1941, the FBI, using primarily census records, compiled the Custodial Detention index ("CDI") on citizens, "enemy" aliens and foreign nationals who might be dangerous, which later led to large-scale internment of Japanese-Americans.  
The bureau recognizes four census regions within the United States, and further organizes them into nine divisions. These regions are groupings of states that subdivide the United States for the presentation of data. They should not be construed as necessarily being thus grouped owing to any geographical, historical, or cultural bonds.
|US Census Regions|
|Region 1: Northeast||Region 2: Midwest||Region 3: South||Region 4: West|
In the last decade, the Census Bureau has begun to rank the states of the Union in qualitative terms based on their quantitative figures so that people could more easily understand the changing dynamics of the country. The goal of this effort was to stir up national pride and understanding along with governmental participation at the state and federal level.