About 12% of all Americans live in California, which ranked first in population among the 50 states in 2002 with an estimated total of 35,116,033, an increase of 3.7% since 2000. California replaced New York as the decennial census leader in 1970, with a total of 19,971,069 residents, and has lengthened its lead ever since. Between 1990 and 2000, California's population grew from 29,760,021 to 33,871,648, an increase of 13.8%. The population is projected to reach 49.3 million by 2025. Los Angeles is the second most populous city in the US, and Los Angeles County ranks first in population among all US counties.

In 2000 the median age was 33.3. Persons under 18 years old accounted for 27.3% of the population while only 10.6% were age 65 or older, lower than the national average of 12.4%

When Europeans first arrived in California, at least 300,000 Indians lived in the area. By 1845, the Indian population had been reduced to about 150,000. Although Spanish missions and settlements were well established in California by the late 18th century, the white population numbered only about 7,000 until the late 1840s. The Gold Rush brought at least 85,000 adventurers to the San Francisco Bay area by 1850, however, and the state's population increased rapidly thereafter. California's population grew to 379,994 by 1860 and had passed the one million mark within 30 years. Starting in 1890, the number of state residents just about doubled every two decades until the 1970s, when the population increased by 18.5%, down from the 27.1% increase of the 1960s. However, the total growth rate during the 1980s was 25.7%, reflecting a population increase of over 6 million.

More than 90% of California's residents live in metropolitan areas. The population density in 2000 was 217.2 persons per sq mi, up from 190.8 per sq mi in 1990. Densities in urban areas were much higher—7,873 per sq mi in Los Angeles and 16,632 per sq mi in San Francisco—the country's second most densely populated city, surpassed only by New York City. In 2000, only 10.6% of all Californians were over 65, lower than the national average of 12.7%.

The majority of Californians live in urban areas; 65% reside in metropolitan San Francisco and Los Angeles. Between 1997 and 2002 the largest population growth occurred mainly in the Central Valley and foothill counties, and in Riverside and San Bernardino Counties in Southern California. The five counties of Los Angeles, Orange, Riverside, San Bernardino, and San Diego accounted for 55% of California's total population in 2002, and 52% of the total increase in population since 1997. The city of Los Angeles had an estimated 2002 population of 3,798,981; San Diego, 1,259,532; San Jose, 894,943; San Francisco, 776,733; Long Beach, 472,412; Fresno, 445,227; Sacramento, 435,245; Oakland, 402,777; Santa Ana, 343,413; and Anaheim, 332,642. The first four all placed among the nation's 15 most populous cities in 2002.

Los Angeles, which expanded irregularly and lacks a central business district, nearly quadrupled its population from 319,000 in 1910 to 1,240,000 in 1930, and then doubled it to 2,479,000 by 1960. A major component of the city's population growth was the upsurge in the number of blacks after World War II, especially between 1960 and 1970, when the number of blacks increased from 335,000 to 504,000, many of them crowded into the deteriorating Watts section.

In 1999, the Los Angeles–Riverside–Orange County urban complex, with a total estimated population of 16,036,587, was the second most populous metropolitan area in the US (after that of New York). Other estimates for that year include the San Francisco–Oakland–San Jose area, 6,873,645; metropolitan San Diego, 2,820,844; and metropolitan Sacramento, 1,741,002.