San Francisco International Airport opened on May 7, 1927, as Mills Field Municipal Airport: 150 acres of tidelands leased from banker Darius Ogden Mills, a single 5,770-foot dirt runway, and 19 passengers in its first month of operation. It is now one of the most consequential airports in North America. Situated on the edge of San Francisco Bay in unincorporated San Mateo County, roughly 13 miles south of downtown San Francisco, the airport's position at the geographic hinge between the North American continent and the Pacific has shaped its entire identity.

SFO is the primary transpacific hub for United Airlines, which holds roughly 45% of the airport's market share and serves 19 Asia-Pacific destinations nonstop, more Pacific Rim service than from any other United hub. Alaska Airlines is also a hub carrier. The airport is the second busiest in California by passenger volume, handling over 54 million passengers in 2025. Its International Terminal, designed by Craig Hartman of Skidmore, Owings and Merrill and opened in December 2000, covers 1.8 million square feet, making it the largest international airport terminal in North America.

The airport's 4 terminals reflect nearly a century of layered growth. Terminal 2, the oldest building still standing, was dedicated in August 1954. Harvey Milk Terminal 1 (originally the South Terminal, opened in 1963 and renamed in 2019) became the first airport terminal in the world named for an LGBTQ leader, and earned LEED Platinum and Fitwel certifications following a $2.4 billion renovation. BART service connecting the airport to San Francisco and the broader Bay Area launched in June 2003, cementing SFO's role as a global hub and an integrated piece of the region's transit network.