Boulder is a Home Rule Municipality that is the county seat and most populous city of Boulder County, Colorado, in the United States. Boulder is the 11th most populous city in the state of Colorado. The United States Census Bureau estimates that in 2005 the population of the city of Boulder was 91,685, while the population of the Boulder Metropolitan Statistical Area was 280,440. Boulder is the home of the University of Colorado at Boulder, the largest university in Colorado and Naropa University, the only accredited Buddhist-inspired university in the United States. Boulder is situated 25 miles (40 km) northwest of Denver at an elevation of 5,430 feet (1,655 m).

The City of Boulder is in Boulder Valley where the Rocky Mountains meet the Great Plains. Just west of the city are imposing slabs of sedimentary stone tilted up on the foothills, known as the Flatirons. The Flatirons are a widely recognized symbol of Boulder.

The primary water flow through the city is Boulder Creek. The creek was named well ahead of the city's founding, for all of the large granite boulders that have cascaded into the creek over the eons. It is from Boulder Creek that Boulder City and hence Boulder is believed to have taken its name. Boulder Creek has significant water flow, derived primarily from snow melt and minor springs west of the city. The creek is a tributary of the South Platte River.