Congressional districts are redrawn every 10 years following the national census. The process of redrawing Congressional districts is called redistricting. The U.S. Constitution actually requires a census every 10 years for the express purpose of redistricting Congressional districts.
Although the U.S. House of Representatives, which is a body subject to redistricting, is on the federal level of government, the U.S. Constitution delegates the duty of determining Congressional districts to members of state legislatures across the United States. Federal guidelines exist regarding the redistricting process. In addition, a number of cases handed down by the U.S. Supreme Court dictate the manner in which redistricting is to occur.