Since 1790, the United States has taken an official headcount to help ensure a government by and for the people. Article I, Section 2 of the United States Constitution mandates this federal census every 10 years. The Census counts every man, woman and child to guarantee that all people are fairly represented in the political process.
For a more thorough background, see this document: History of Redistricting and Reapportionment in Washington State.
State constitutional citation for redistricting: Article II, Section 43
Statutory citation for redistricting: RCW 44.05
For the archive of the 2011 redistricting effort, see: 2011 Redistricting Web Site.