Arapahoe County is the third most populous of the 64 counties of the state of Colorado of the United States. The United States Census Bureau estimated that the county population was 595,546 in 2012, a 4.1% increase since 2010 census. The county seat is Littleton; the most populous city is Aurora. Arapahoe County is part of the Denver–Aurora Metropolitan Statistical Area and the Denver–Aurora–Boulder Combined Statistical Area. Arapahoe County calls itself "Colorado's First County" since its origins predate the Pike's Peak Gold Rush.