Lassen is one of the 58 counties in the state of California. It was created in 1864 from parts of Plumas and Shasta counties, as well as parts of the old Lake County, Nevada. As of the 2010 census the population was 34,895, up from 33,828 in 2000. The county seat and only city is Susanville.
It covers 4,720.37 square miles (12,225.7 km2), of which 4,557.27 square miles (11,803.3 km2) is land. It is bordered by Sierra County to the southeast, Plumas County to the south, Shasta County to the west, Modoc County to the north and Washoe County, Nevada to the east.