Pittsburgh (/ˈpɪtsbərɡ/PITS-burg) is a city in the Commonwealth of Pennsylvania in the United States, and is the county seat of Allegheny County. The Combined Statistical Area (CSA) population of 2,659,937 is the largest in both the Ohio Valley and Appalachia, the second-largest in Pennsylvania after Philadelphia and the 20th-largest in the U.S. Located at the confluence of the Allegheny and Monongahela rivers, which form the Ohio River, Pittsburgh is known as both "the Steel City" for its more than 300 steel-related businesses, and as the "City of Bridges" for its 446 bridges. The city features 30 skyscrapers, two inclines, a pre-revolutionary fortification and the Point State Park at the confluence of the rivers. The city developed as a vital link of the Atlantic coast and Midwest. The mineral-rich Allegheny Mountains made the area coveted by the French and British empires, Virginia, Whiskey Rebels, and Civil War raiders.
Aside from steel, Pittsburgh has led in manufacturing of aluminum, glass, shipbuilding, petroleum, foods, sports, transportation, computing, autos, and electronics. For part of the 20th century, Pittsburgh was behind only New York and Chicago in corporate headquarters employment; it had the most U.S. stockholders per capita. America's 1980s deindustrialization laid off area blue-collar workers and thousands of downtown white-collar workers when the longtime Pittsburgh-based world headquarters of Gulf Oil, Sunbeam, Rockwell and Westinghouse moved out. This heritage left the area with renowned museums, medical centers, parks, research centers, libraries, a diverse cultural district and the most bars per capita in the U.S. In 2015, Pittsburgh was listed among the "eleven most livable cities in the world"; The Economist's Global Liveability Ranking placed Pittsburgh as the first- or second-most livable city in the United States in 2005, 2009, 2011, 2012 and 2014.
Google, Apple, Bosch, Facebook, Uber, Nokia, Autodesk, and IBM are among 1,600 technology firms generating $20.7 billion in annual Pittsburgh payrolls. The area has served also as the long-time federal agency headquarters for cyber defense, software engineering, robotics, energy research and the nuclear navy. The area is home to 68 colleges and universities, including research and development leaders Carnegie Mellon University and the University of Pittsburgh. The nation's fifth-largest bank, eight Fortune 500 companies, and six of the top 300 US law firms make their global headquarters in the Pittsburgh area, while RAND, BNY Mellon, Nova, FedEx, Bayer and NIOSH have regional bases that helped Pittsburgh become the sixth-best area for U.S. job growth.
The region is a hub for Leadership in Energy and Environmental Design, sustainable energy, and energy extraction.