According to U.S. Census data, in 2013 the most populous county in California was Los Angeles County, followed by San Diego County and Orange County. Riverside County, San Bernardino County and Santa Clara County are the next three most-populated.
All six of these counties make the top-20 list of most populated counties in the United States overall. Los Angeles tops the list overall for both the state and the country, with a population of over 10 million people according to the 2013 figures. San Diego and Orange County, in contrast, had only a little over 3 million people apiece, despite making the top-10 for the country's most-populated counties.