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 22 federal censuses since that time. The current national census is being held in 2010 and the next census is scheduled for 2020. For years between the decennial censuses, the Census Bureau issues estimates made using surveys and statistical models, in particular, the American Community Survey.

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. The Census Bureau bases its decision about whom to count on the concept of usual residence. Usual residence, a principle established by the Census Act of 1790, is defined as the place a person lives and sleeps most of the time. The Census Bureau uses special procedures to ensure that those without conventional housing are counted; however, data from these operations are not considered as accurate as data obtained from traditional procedures. The practice of including non-citizens in the official census figures is controversial because 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 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. Certain American citizens living overseas are specifically excluded from being counted in the census even though they may vote. Only Americans living abroad who are "Federal employees (military and civilian) and their dependents living overseas with them" are counted. "Private U.S. citizens living abroad who are not affiliated with the Federal government (either as employees or their dependents) will not be included in the overseas counts. These overseas counts are used solely for reapportioning seats in the U. S. House of Representatives."