The United States is an urbanized nation, with 80.6 percent of its population residing in cities and suburbs. The mean population center of the United States has consistently shifted westward and southward, with California and Texas currently the most populous states. According to Californians for Population Stabilization, U.S. population growth is now the highest among developed countries. Births, supplemented by immigration, help to offset the aging population. The total fertility rate in the United States estimated for 2007 is 2.09, which is roughly the replacement level for industrialized countries. The total U.S. population crossed the 100 million mark around 1915, the 200 million mark in 1967, and the 300 million mark in 2006 (on Tuesday, October 17). The U.S. population more than tripled during the 20th century - a growth rate of about 1.3 percent a year - from about 76 million in 1900 to 281 million in 2000. This is unlike most European countries, especially Germany, Russia, Italy and Greece, whose populations are slowly declining, and whose fertility rates are below replacement.

According to U.S. Census Bureau's estimation for 2005, 45% of American children under the age of 5 are minorities. In 2006, the nation’s minority population reached 100.7 million. A year before, the minority population totaled 98.3 million. Hispanics accounted for almost half (1.4 million) of the national population growth of 2.9 million between July 1, 2005, and July 1, 2006. In thirty-five of the country's fifty largest cities, white people are or soon will be in the minority.