There are 18 islands scattered throughout the San Francisco Bay, including uninhabited rocks, protected wildlife refuges, and developed islands with sizable populations. Some of the islands of the San Francisco Bay are quite famous, as is the case with Alcatraz. Many are accessible only by private watercraft such as sailboats, and each has unique features which make it worth visiting, if you can get there.
Many of the islands are set aside as parks and refuges. Angel Island is a state park accessible by ferry from several locations around the bay, and Alcatraz is a national park. Visitors to these islands are welcome year round, although summertime is the peak season for visiting. There are also several wildlife refuges, including the East and West Marin Islands, Brooks Island, and Bair Island. Brooks Island is part of the East Bay Regional Parks District, and hosts migrant birds, wetlands, and walking paths. Bair Island is an extensive wildlife refuge in the South Bay, and is an important part of the Bay Area ecology.
There are also several islands of the San Francisco Bay that are little more than glorified rocky outcroppings. These include Red Rock Island, on the western side of the Richmond Bridge, along with the East and West Sisters, Ballena Bay Island next to Alameda, and Pat Rock. Red Rock Island is also the only privately owned of the islands in the San Francisco Bay, although the fact that it technically lies in three counties makes potential development a challenge.
There are also inhabited islands in the bay. One, Coast Guard Island, is set aside for military use only. The other inhabited islands are all open to the public. Yerba Buena Island lies in the middle of the San Francisco Bay Bridge, and hosts a civilian population along with a small Coast Guard station. Treasure Island, an artificial island attached to Yerba Buena by a causeway, is a former Naval base with civilian housing located at its northern end. Alameda, an island directly off the coast of Oakland, is accessible via a number of causeways and has a sizable collection of examples of Victorian and Queen Anne architecture.
These islands, as can be seen, are quite varied. Many Bay area residents are only capable of naming a handful of the 18 islands, so prepare to impress people with your local knowledge. The varied residences and nature preserves on the islands of San Francisco bay also provide an interesting cross section of Bay area life, both human and animal.