() is a Home Rule Municipality in Boulder County, Colorado, United States. The population was 18,937 at the 2000 census. Louisville began as a rough mining community in 1877, suffered through a period of extraordinary labor violence early in the 20th century, and then, when the mines closed in the 1950s, made a transition to suburban residential community.
As of the census of 2000, there were 18,937 people, 7,216 households, and 4,950 families residing in the city. The population density was 2,223.6 people per square mile (858.2/km²). There were 7,389 housing units at an average density of 867.6/sq mi (334.8/km²). The racial makeup of the city was 91.17% White, 0.93% African American, 0.54% Native American, 3.55% Asian, 0.08% Pacific Islander, 1.83% from other races, and 1.90% from two or more races. Hispanic or Latino of any race were 5.02% of the population.
There were 7,216 households out of which 40.5% had children under the age of 18 living with them, 56.3% were married couples living together, 8.6% had a female householder with no husband present, and 31.4% were non-families. 22.4% of all households were made up of individuals and 4.6% had someone living alone who was 65 years of age or older. The average household size was 2.61 and the average family size was 3.13.
In the city the population was spread out with 28.7% under the age of 18, 6.3% from 18 to 24, 35.6% from 25 to 44, 23.3% from 45 to 64, and 6.1% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 98.6 males. For every 100 females age 18 and over, there were 95.0 males.
The median income for a household in the city was $69,945, and the median income for a family was $81,512. Males had a median income of $57,159 versus $36,659 for females. The per capita income for the city was $31,828. 3.0% of the population and 2.1% of families were below the poverty line. 2.2% of those under the age of 18 and 5.6% of those 65 and older were living below the poverty line.