San Diego County is a county located in the southwestern corner of the state of California, in the United States. 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 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 Metropolitan Statistical Area. The San Diego-Carlsbad, CA Metropolitan Statistical Area as the 17th most populous metropolitan statistical area and the 18th most populous primary statistical area of the United States as of July 1, 2012. San Diego is also part of the San Diego–Tijuana metropolitan area, the largest metropolitan area shared between the United States and Mexico. Greater San Diego ranks as the 38th largest metropolitan area in the Americas.
San Diego County has 70 miles of coastline. 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.