Every ten years the Federal Government conducts a census to count the number of people living in the United States. The census is mandated by Article 1 Sec. 2 of the United States Constitution as amended in 1868.
The method of conducting the census is regulated by Title 13 of the Code of Federal Regulations.
The most important consequence of the census is the apportionment of representatives for each state. 435 representatives are divided among the 50 states according to how many people live in each state, this number being derived from the census.
Richardson Library has Decennial Census records from 1910 to present. More Decennial census data is available at NHGIS/IPUMS.