Pierce County is notable for being home to the Mount Rainier volcano, the tallest mountain in the Cascade Range. Its most recent recorded eruption was between 1820 and 1854. There is no imminent risk of eruption, but geologists expect that the volcano will erupt again. If this should happen, parts of Pierce County and the Puyallup Valley would be at risk from lahars, lava, or pyroclastic flows. The Mount Rainier Volcano Lahar Warning System was established in 1998 to assist in the evacuation of the Puyallup River valley in case of eruption.
As of the census of 2000, there were 700,820 people, 260,800 households, and 180,212 families residing in the county. The population density was 417 people per square mile (161/km²). There were 277,060 housing units at an average density of 165 per square mile (64/km²). The racial makeup of the county was 78.39% White, 6.95% Black or African American, 1.42% Native American, 5.08% Asian, 0.85% Pacific Islander, 2.20% from other races, and 5.11% from two or more races. 5.51% of the population were Hispanic or Latino of any race. 16.1% were of German, 8.6% Irish, 8.2% English, 6.3% United States or American and 6.2% Norwegian ancestry according to Census 2000.
There were 260,800 households out of which 35.90% had children under the age of 18 living with them, 52.80% were married couples living together, 11.80% had a female householder with no husband present, and 30.90% were non-families. 24.30% of all households were made up of individuals and 7.60% had someone living alone who was 65 years of age or older. The average household size was 2.60 and the average family size was 3.10.
In the county, the population was spread out with 27.20% under the age of 18, 9.80% from 18 to 24, 31.30% from 25 to 44, 21.50% from 45 to 64, and 10.20% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 98.90 males. For every 100 females age 18 and over, there were 96.70 males.
The median income for a household in the county was $45,204, and the median income for a family was $52,098. Males had a median income of $38,510 versus $28,580 for females. The per capita income for the county was $20,948. About 7.50% of families and 10.50% of the population were below the poverty line, including 13.20% of those under age 18 and 7.20% of those age 65 or over.