Sunnyvale (/ˈsʌniveɪl/ or /ˈsʌnivəl/), officially the City of Sunnyvale, is a city located in Santa Clara CountyCalifornia. As of the 2010 United States Census, the population was 140,095.

The city is one of the major cities that make up Silicon Valley. It is the seventh most populous city in the San Francisco Bay Area. Sunnyvale is bordered by portions of San Jose to the north, Moffett Federal Airfield to the northwest, Mountain View to the west, Los Altos to the southwest, Cupertino to the south, and Santa Clara to the east. It lies along the historic El Camino Real and Highway 101.

As part of California's high-tech area known as Silicon Valley, companies such as Juniper NetworksFortinetAMDNetAppSpansionYahoo!AppliedMicro and Ariba are headquartered there. Sunnyvale is also home to several aerospace/defense companies; Lockheed Martin has a major facility in Sunnyvale, and HoneywellNorthrop Grumman Electronic Systems - Marine Systems (formerly Joshua Hendy Iron Works), Finisar, and Spirent also have offices in Sunnyvale. Sunnyvale was also the home to Onizuka Air Force Station, often referred to as "the Blue Cube" due to the color and shape of its windowless main building. The facility, previously known as Sunnyvale Air Force Station, was named for the deceased Space Shuttle Challenger astronaut Ellison Onizuka. It served as an artificial satellite control facility of the United States armed forces until August 2010 and has since been decommissioned and demolished.

Sunnyvale is one of the few U.S. cities to have a single unified Department of Public Safety, where all personnel are trained as firefighterspolice officers, and EMTs, so they can respond to an emergency in any of the three roles.

Library services for the city are provided by the Sunnyvale Public Library, located at the Sunnyvale Civic Center.