San Diego County is a county located in the southwestern corner of the state of California. As of the 2010 census, the population was 3,095,313 making it the second-most populous county in California and the fifth-most populous county in the United States. Its county seat is San Diego, the eighth-most populous city in the United States. It is the south-westernmost county in the 48 contiguous United States.
San Diego County comprises the San Diego-Carlsbad, CA Metropolitan Statistical Area. It is also part of the San Diego–Tijuana metropolitan area, the largest metropolitan area shared between the United States and Mexico. San Diego County has 70 miles of coastline. The county has over 4,200 square miles of land area. Most of the county has a mild Mediterranean climate to semiarid climate, though there are mountains that receive frost and snow in the wintertime.
There are also 16 naval and military installations of the U.S. Navy, U.S. Marine Corps, and the U.S. Coast Guard in San Diego County. These include the Naval Base San Diego, Marine Corps Base Camp Pendleton, Marine Corps Air Station Miramar, and Naval Air Station North Island.
Arising from an effort by the state government to identify regional economies, San Diego County and Imperial County are part of the Southern Border Region, one of nine such regions. As a regional economy, the Southern Border Region is the smallest but most economically diverse region in the state. However, the two counties maintain weak relations and have little in common aside from their common border.
From north to south, San Diego County extends from the southern borders of Orange County and Riverside County to the Mexico–United States border and Baja California. From west to east, San Diego County stretches from the Pacific Ocean to its boundary with Imperial County.