Reapportionment
The results of the census are used to determine how many congressional districts each state is apportioned. Congress defines the formula, in accordance with Title 2 of the U.S. Code, to reapportion among the states the 435 seats in the United States House of Representatives. The apportionment population consists of the resident population of the fifty states, plus the overseas military and federal civilian employees and their dependents living with them who could be allocated to a state. Each member of the House represents a population of about 647,000. The populations of the District of Columbia and Puerto Rico are excluded from the apportionment population because they do not have voting seats in the U. S. House of Representatives.
Since the first census in 1790, the decennial count has been the basis for the United States representative form of government. Article I, Section II specifies that "The Number of Representatives shall not exceed one for every thirty Thousand, but each State shall have at Least one Representative." In 1790, each member of the House represented about 34,000 residents. Since then, the House more than quadrupled in size, and in 1911 the number of representatives was fixed at 435. Today, each member represents about 20 times as many constituents.
Read more about this topic: 2000 United States Census