San Diego (pop. 1,305,736 as of 2005 ) is a coastal Southern California city located in the southwestern corner of the continental United States. It is the second largest city in California and the seventh largest in the United States . It lies just north of the Mexican border (shares border with Tijuana, Mexico), and is a home for United States Navy and Marine Corps bases, many miles of beaches, and a mild marine climate. The annual mean temperature is 64.4 degrees Fahrenheit (18°C). San Diego's economy centers on tourism, trade, agriculture, ship-building, the military, biotechnology, computer science and electronics.

The University of California, San Diego and nearby research institutes on Torrey Mesa provide a base for technological innovation and there are numerous high-tech and biotech companies in the area, such as Qualcomm, Neurocrine, and Illumina. Major tourists attactions include the cities beaches and bays, Balboa Park with its many museums, the San Diego Zoo, Sea World, San Diego Wild Animal Park (in Escondido, north of the city) and Old Town, the site of of the original Spanish settlement.

Downtown San Diego is located on San Diego Bay. Coronado Island and Point Loma separate the bay from the ocean. Ocean Beach is on the west side of Point Loma. Mission Beach and Pacific Beach lie between the ocean and Mission Bay, a man-made aquatic park. La Jolla, an affluent community, lies north of Pacific Beach. Mount Soledad in La Jolla offers views from northern San Diego County to Mexico.

Mountains rise to the east of the city, and beyond the mountains are desert areas. Cleveland National Forest is a half-hour drive from downtown San Diego. Numerous farms are found in the valleys northeast of the city. The city of San Diego itself has deep canyons separating its mesas, creating small pockets of natural parkland scattered throughout the city.

Military bases in or near San Diego include U.S. Navy ports, Marine Corps bases, and Coast Guard stations. San Diego is the home port of the largest naval fleet in the world, including two Navy supercarriers (the USS Nimitz and the USS Ronald Reagan) five amphibious assault ships, several Los Angeles-class submarines, and many smaller ships.

One of the Marine Corps' two Recruit Depots is located here. San Diego is also known as the "birthplace of naval aviation," although Pensacola, Florida makes a rival claim.

Four Navy vessels have been named USS San Diego in honor of the city.