As mandated by the U.S. Constitution, America gets just one chance each decade to count its population. The U.S. Census Bureau counts every resident in the United States. It is mandated by Article I, Section 2 of the U.S. Constitution and takes place every 10 years.
Not only do we have to count people, we have to know where they live so we can provide accurate population numbers for counties, states, municipios, etc. These numbers are used to draw school and voting districts.