The Sierra Club is an environmental organization with chapters in all 50 United States, Washington D.C., and Puerto Rico. The club was founded on May 28, 1892 in San Francisco, California by Scottish-American preservationist John Muir, who became the first president as well as the longest serving president, at approximately 20 years in this leadership position. The Sierra Club operates only in the United States and holds the legal status of 501(c)(4) nonprofit social welfare organization. Sierra Club Canada is a separate entity.

Traditionally associated with the progressive movement, the club was one of the first large-scale environmental preservation organizations in the world, and currently engages in lobbying politicians to promote environmentalist policies. Recent focuses of the club include promoting sustainable energy and mitigating global warming, as well as opposition to the use of coal, hydropower and nuclear power. The club is known for its political endorsements, which are often sought after by candidates in local elections; it generally supports liberal and progressive candidates in elections.

The Sierra Club is organized on both a national and state level with chapters named for the 50 states and two U.S. territories (Puerto Rico and Washington D.C.) California is the lone state to have numerous chapters named for California counties. The club chapters allow for regional groups and committees, some of which have many thousands of members. These chapters further allow for special interest sections (ex. Camera, Outings), committees (conservation and political), and task forces on a single issue with some kind of geography involved. While much activity is coordinated at a local level, the club is a unified organization; decisions made at the national level take precedence, including the removal and creation of Chapters, as well as recruiting and removing members.

In addition to political advocacy, the Sierra Club organizes outdoor recreation activities, and has historically been a notable organization for mountaineering and rock climbing in the United States. Members of the Sierra Club pioneered the Yosemite Decimal System of climbing, and were responsible for a substantial amount of the early development of climbing. Much of this activity occurred in the group's namesake Sierra Nevada. The Sierra Club does not set standards for or regulate alpinism, but it organizes wilderness courses, hikes, and occasional alpine expeditions for members. In California, the club, through its outdoor recreation groups, is usually considered the state's analogue to other state mountaineering clubs such as Mazamas, The Mountaineers, or the Colorado Mountain Club.

San Francisco (; Spanish for "Saint Francis"), officially the City and County of San Francisco, is a cultural, commercial, and financial center in the U.S. state of California. Located in Northern California, San Francisco is the 17th most populous city proper in the United States, and the fourth most populous in California, with 873,965 residents as of 2020. It covers an area of about 46.9 square miles (121 square kilometers), mostly at the north end of the San Francisco Peninsula in the San Francisco Bay Area, making it the second most densely populated large U.S. city, and the fifth most densely populated U.S. county, behind only four of the five New York City boroughs. San Francisco is the 12th-largest metropolitan statistical area in the United States with 4.7 million residents, and the fourth-largest by economic output, with a GDP of $592 billion in 2019. With San Jose, it forms the San Jose–San Francisco–Oakland, CA Combined Statistical Area, the fifth most populous combined statistical area in the United States, with 9.6 million residents as of 2019. Colloquial nicknames for San Francisco include SF, San Fran, The City, and Frisco.In 2019, San Francisco was the county with the seventh-highest income in the United States, with a per capita income of $139,405. In the same year, San Francisco proper had a GDP of $203.5 billion, and a GDP per capita of $230,829. The San Jose–San Francisco–Oakland, CA Combined Statistical Area, with a GDP of $1.09 trillion as of 2019, is the country's third-largest economy. Of the 105 primary statistical areas in the U.S. with over 500,000 residents, this CSA had the highest GDP per capita in 2019, at $112,348. San Francisco was ranked 5th in the world and second in the United States on the Global Financial Centres Index as of September 2021.San Francisco was founded on June 29, 1776, when colonists from Spain established the Presidio of San Francisco at the Golden Gate and Mission San Francisco de Asís a few miles away, both named for Francis of Assisi. The California Gold Rush of 1849 brought rapid growth, making it the largest city on the West Coast at the time; between 1870 and 1900, approximately one quarter of California's population resided in the city proper. In 1856, San Francisco became a consolidated city-county. After three-quarters of the city was destroyed by the 1906 earthquake and fire, it was quickly rebuilt, hosting the Panama-Pacific International Exposition nine years later. In World War II, it was a major port of embarkation for service members shipping out to the Pacific Theater. It then became the birthplace of the United Nations in 1945. After the war, the confluence of returning servicemen, significant immigration, liberalizing attitudes, the rise of the "beatnik" and "hippie" countercultures, the Sexual Revolution, the Peace Movement growing from opposition to United States involvement in the Vietnam War, 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. Politically, the city votes strongly along liberal Democratic Party lines.

A popular tourist destination, San Francisco is known for its cool summers, fog, steep rolling hills, eclectic mix of architecture, and landmarks, including the Golden Gate Bridge, cable cars, the former Alcatraz Federal Penitentiary, Fisherman's Wharf, and its Chinatown district. San Francisco is also the headquarters of companies such as Wells Fargo, Twitter, Block, Airbnb, Levi Strauss & Co., Gap Inc., Salesforce, Dropbox, Pacific Gas and Electric Company, Uber, and Lyft. The city, and the surrounding Bay Area, is a global center of the sciences and arts and is home to a number of educational and cultural institutions, such as the University of California, San Francisco (UCSF), the University of San Francisco (USF), San Francisco State University (SFSU), the de Young Museum, the San Francisco Museum of Modern Art, the SFJAZZ Center, the San Francisco Symphony and the California Academy of Sciences. More recently, statewide droughts in California have strained the city's water security.