Both the California Department of Finance and the U.S. Bureau of the Census produce population estimates for California. The Department of Finance's latest estimate, released in May 2007, stated that there were almost 37.7 million people living in California as of January 1, 2007. To view the actual numbers with county breakouts.
The Bureau of the Census estimated that there were just under 36.5 million people living in California as of July 1, 2006. To view the actual numbers with county breakouts.
The two agencies use different methods to calculate their estimates, so you should consult their Web sites to learn the differences if need be.