San Francisco, officially the City and County of San Francisco, is the fourth most populous city in California and the 12th most populous city in the United States, with a 2009 estimated population of 815,358. The only consolidated city-county in California, it encompasses a land area of 46.7 square miles (121 km2) on the northern end of the San Francisco Peninsula, giving it a density of 17,323 people/mi² (6,688.4 people/km²). It is the most densely-settled large city (population greater than 200,000) in the state of California and the second-most densely populated large city in the United States. San Francisco is the financial, cultural, and transportation center of the San Francisco Bay Area, a region of more than 7.4 million people.

In 1776, the Spanish established a fort at the Golden Gate and a mission named for Francis of Assisi on the site. The California Gold Rush in 1848 propelled the city into a period of rapid growth, increasing the population in one year from 1,000 to 25,000, and thus transforming it into the largest city on the West Coast at the time. After three-quarters of the city was destroyed by the 1906 earthquake and fire, San Francisco was quickly rebuilt, hosting the Panama-Pacific International Exposition nine years later. During World War II, San Francisco was the port of embarkation for service members shipping out to the Pacific Theater. After the war, the confluence of returning servicemen, massive immigration, liberalizing attitudes, and other factors led to the Summer of Love and the gay rights movement, cementing San Francisco as a center of liberal activism in the United States.

Today, San Francisco is a popular international tourist destination, renowned for its chilly summer fog, steep rolling hills, eclectic mix of Victorian and modern architecture and its famous landmarks, including the Golden Gate Bridge, cable cars, and Chinatown. The city is also a principal banking and finance center, and the home to more than 30 international financial institutions, helping to make San Francisco eighteenth place in the world's top producing cities, ninth in the United States, and fifteenth place in the top twenty Global Financial Centers.