Aurora is a Home Rule Municipality spanning Arapahoe, Adams, and Douglas counties in Colorado. The city is the third most populous city in the state of Colorado and the 59th most populous city in the United States. The city had an estimated population of 325,078 in the 2010 Census. Denver and Aurora are the principal cities of the Denver-Aurora-Broomfield, CO Metropolitan Statistical Area, which in 2007 had an estimated population of 2,464,866. (22nd most populous MSA) However, Denver and Aurora combined make up less than half of the Denver-Aurora-Broomfield Metropolitan Statistical Area's population and Aurora has approximately half the population of Denver. The estimated population of the Denver-Aurora-Boulder Combined Statistical Area was 2,998,878 in 2007(15th most populous CSA).