Solano is one of the 58 counties in the state of California. It was created in 1850, making it one of the original 27 counties in the state of California. As of the 2010 census, the population was 413,344, up from 394,542 in 2000. The county seat is Fairfield and the largest city is Vallejo.
The county has an area of 906.67 square miles (2,348.3 km2), of which 829.19 square miles (2,147.6 km2) (or 91.45%) is land. Solano is bordered on the west by Napa and Sonoma counties, on the north by Yolo County, and on the east by Sacramento County. It is also connected by bridge with Contra Costa County to the south.
Solano County is one of the nine traditional Bay Area counties.