Increasing U.S. energy supply diversity has been the goal of many Presidents and Congresses. This commitment has been prompted by concerns about national security, the environment, and the U.S. balance of payments. Investments in new energy sources also have been seen as a way to expand domestic manufacturing. For all of these reasons, the federal government has a variety of policies to promote wind power. Expanding the use of wind energy requires installation of wind turbines. These are complex machines composed of some 8,000 components, created from basic industrial materials such as steel, aluminum, concrete, and fiberglass. Major components in a wind turbine include the rotor blades, a nacelle and controls (the heart and brain of a wind turbine), a tower, and other parts such as large bearings, transformers, gearboxes, and generators. Turbine manufacturing involves an extensive supply chain. Until recently, Europe has been the hub for turbine production, supported by national renewable energy deployment policies in countries such as Denmark, Germany, and Spain. However, support for renewable energy including wind power has begun to wane across Europe as governments there reduce or remove some subsidies. Competitive wind turbine manufacturing sectors are also located in India and Japan and are emerging in China and South Korea. U.S. and foreign manufacturers have expanded their capacity in the United States to assemble and produce wind turbines and components. About 470 U.S. manufacturing facilities produced wind turbines and components in 2011, up from as few as 30 in 2004. An estimated 30,000 U.S. workers were employed in the manufacturing of wind turbines in 2011. Because turbine blades, towers, and certain other components are large and difficult to transport, manufacturing clusters have developed in certain states, notably Colorado, Iowa, and Texas, which offer proximity to the best locations for wind energy production. The U.S. wind turbine manufacturing industry also depends on imports, with the majority coming from European countries, where the technical ability to produce large wind turbines was developed. Although turbine manufacturers' supply chains are global, recent investments are estimated to have raised the share of parts manufactured in the United States to 67% in 2011, up from 35% in 2005-2006. The outlook for wind turbine manufacturing in the United States is more uncertain now than in recent years. For the past two decades, a variety of federal laws and state policies have encouraged both wind energy production and the use of U.S.-made equipment to generate that energy. One apparent challenge for the industry is the scheduled expiration at year-end 2012 of the production tax credit (PTC), which the industry claims could reduce domestic turbine sales to zero in 2013. In anticipation, at least a dozen wind turbine manufacturers announced layoffs or hiring freezes at their U.S. facilities in 2012, citing uncertainty around the renewal of the PTC as one reason. Other factors affecting the health of the U.S. wind industry are intense price competition from natural gas, an oversupply in wind turbines, and softening demand for renewable electricity.