Placer County (// PLAS-ər), officially the County of Placer, is a county located in both the Sacramento Valley and Sierra Nevada regions of the U.S. state of California, in what is known as the Gold Country. It stretches from the suburbs of Sacramento to Lake Tahoe and the Nevada border. Because of the expansion of the Sacramento metropolitan area, Placer County is one of the fastest growing counties in California by population. Between 2000 and 2010, the population grew from 248,399 to 348,432. The county seat is Auburn.