San Diego is the eighth-largest city in the United States and second-largest city in California. The city is located on the coast of the Pacific Ocean in Southern California, immediately adjacent to the Mexican border. The birthplace of California, San Diego is known for its mild year-round climate, its natural deep-water harbor, and its long association with the U.S. Navy. The population was 1,301,617 at the 2010 census.
 
Historically home to the Kumeyaay people, San Diego was the first site visited by Europeans on what is now the West Coast of the United States. Upon landing in San Diego Bay in 1542, Juan Cabrillo claimed the entire area for Spain, forming the basis for the settlement of Alta California 200 years later. The Presidio and Mission of San Diego, founded in 1769, were the first European settlement in what is now California. In 1821, San Diego became part of newly independent Mexico, and in 1850, became part of the United States following the Mexican-American War and the admission of California to the union.
 
The city is the county seat of San Diego County and is the economic center of the San Diego–Carlsbad–San Marcos metropolitan area as well as the San Diego–Tijuana metropolitan area. San Diego's main economic engines are military and defense-related activities, tourism, international trade, and manufacturing. The presence of the University of California, San Diego (UCSD), with the affiliated UCSD Medical Center, has helped make the area a center of research in biotechnology.

The area of San Diego has been inhabited for more than 10,000 years by the Kumeyaay people. The first European to visit the region was Portuguese-born explorer Juan Rodríguez Cabrillo sailing under the flag of Castile. Sailing his flagship San Salvador from Navidad, New Spain, Cabrillo claimed the bay for the Spanish Empire in 1542 and named the site 'San Miguel'. In November 1602, Sebastián Vizcaíno was sent to map the California coast. Arriving on his flagship San Diego, Vizcaíno surveyed the harbor and what are now Mission Bay and Point Loma and named the area for the Catholic Saint Didacus, a Spaniard more commonly known as San Diego de Alcalá. On November 12, 1602, the first Christian religious service of record in Alta California was conducted by Friar Antonio de la Ascensión, a member of Vizcaíno's expedition, to celebrate the feast day of San Diego.
 
In May 1769, Gaspar de Portolà established the Fort Presidio of San Diego on a hill near the San Diego River. In July of the same year, Mission San Diego de Alcalá was founded by Franciscan friars under Father Junípero Serra. By 1797, the mission boasted the largest native population in Alta California, with over 1,400 neophytes living in and around the mission proper. Mission San Diego was the southern anchor in California of the historic mission trail El Camino Real. Both the Presidio and the Mission are National Historic Landmarks.
 
In 1821, Mexico won its independence from Spain, and San Diego became part of the Mexican state of Alta California. The fort on Presidio Hill was gradually abandoned, while the town of San Diego grew up on the level land below Presidio Hill. The Mission was secularized by the Mexican government, and most of the Mission lands were distributed to wealthy Californio settlers.
 
As a result of the Mexican-American War of 1846–1848, the territory of Alta California, including San Diego, was ceded to the United States by Mexico. The Battle of San Pasqual, a battle of the Mexican-American War, was fought in 1846 in the San Pasqual Valley which is now part of the city of San Diego. The state of California was admitted to the United States in 1850. That same year San Diego was designated the seat of the newly established San Diego County and was incorporated as a city. The initial city charter was established in 1889 and today's city charter was adopted in 1931.
 
The original town of San Diego was located at the foot of Presidio Hill, in the area which is now Old Town San Diego State Historic Park. The location was not ideal, being several miles away from navigable water. In the late 1860s, Alonzo Horton promoted a move to "New Town", several miles south of the original settlement, in the area which became Downtown San Diego. People and businesses flocked to New Town because of its location on San Diego Bay convenient to shipping. New Town quickly eclipsed the original settlement, known to this day as Old Town, and became the economic and governmental heart of the city.
 
In the early part of the 20th century, San Diego hosted two World's Fairs: the Panama-California Exposition in 1915 and the California Pacific International Exposition in 1935. Both expositions were held in Balboa Park, and many of the Spanish/Baroque-style buildings that were built for those expositions remain to this day as central features of the park. The buildings were intended to be temporary structures, but most remained in continuous use until they progressively fell into disrepair. Most were eventually rebuilt, using castings of the original facades to retain the architectural style. The menagerie of exotic animals featured at the 1915 exposition provided the basis for the San Diego Zoo.
 
Significant U.S. Navy presence began in 1901 with the establishment of the Navy Coaling Station in Point Loma, and expanded greatly during the 1920s. By 1930 the city was host to Naval Base San Diego, Naval Training Center San Diego, San Diego Naval Hospital, Camp Matthews, and Camp Kearny (now Marine Corps Air Station Miramar). The city was also an early center for aviation: as early as World War I San Diego was proclaiming itself "The Air Capital of the West." The city was home to important airplane developers and manufacturers like Ryan Airlines (later Ryan Aeronautical), founded in 1925, and Consolidated Aircraft (later Convair), founded in 1923. Charles A. Lindbergh's plane The Spirit of St. Louis was built in San Diego in 1927 by Ryan Airlines.
 
During World War II, San Diego became a major hub of military and defense activity, due to the presence of so many military installations and defense manufacturers. The city's population grew rapidly during and after World War II, more than doubling between 1930 (147,995) and 1950 (333,865). After World War II, the military continued to play a major role in the local economy, but post-Cold War cutbacks took a heavy toll on the local defense and aerospace industries. The resulting downturn led San Diego leaders to seek to diversify the city's economy by focusing on research and science, as well as tourism.
 
Downtown San Diego was in decline in the 1960s and 1970s but experienced some urban renewal since the early 1980s, including the opening of Horton Plaza, the revival of the Gaslamp Quarter, and the construction of the San Diego Convention Center; Petco Park opened in 2004.