A wind turbine is a device that converts the kinetic energy of wind into electrical energy. As of 2020^[update], hundreds of thousands of large turbines, in installations known as wind farms, were generating over 650 gigawatts of power, with 60 GW added each year. Wind turbines are an increasingly important source of intermittent renewable energy, and are used in many countries to lower energy costs and reduce reliance on fossil fuels. One study claimed that, as of 2009,^[update] wind had the "lowest relative greenhouse gas emissions, the least water consumption demands and the most favorable social impacts" compared to photovoltaic, hydro, geothermal, coal and gas energy sources.

Smaller wind turbines are used for applications such as battery charging and remote devices such as traffic warning signs. Larger turbines can contribute to a domestic power supply while selling unused power back to the utility supplier via the electrical grid.

Wind turbines are manufactured in a wide range of sizes, with either horizontal or vertical axes, though horizontal is most common.

The windwheel of Hero of Alexandria (10–70 CE) marks one of the first recorded instances of wind powering a machine. However, the first known practical wind power plants were built in Sistan, an Eastern province of Persia (now Iran), from the 7th century. These panemone windmills were vertical-axle windmills, which had long vertical drive shafts with rectangular blades. Made of six to twelve sails covered in reed matting or cloth material, these windmills were used to grind grain or draw up water, and were used in the gristmilling and sugarcane industries.

Wind power first appeared in Europe during the Middle Ages. The first historical records of their use in England date to the 11th and 12th centuries; there are reports of German crusaders taking their windmill-making skills to Syria around 1190. By the 14th century, Dutch windmills were in use to drain areas of the Rhine delta. Advanced wind turbines were described by Croatian inventor Fausto Veranzio in his book Machinae Novae (1595). He described vertical-axis wind turbines with curved or V-shaped blades.

The first electricity-generating wind turbine was installed by the Austrian Josef Friedländer at the Vienna International Electrical Exhibition in 1883. It was a Halladay windmill for driving a dynamo. Friedländer's 6.6 m (22 ft) diameter Halladay "wind motor" was supplied by U.S. Wind Engine & Pump Co. of Batavia, Illinois. The 3.7 kW (5 hp) windmill drove a dynamo at ground level that fed electricity into a series of batteries. The batteries powered various electrical tools and lamps, as well as a threshing machine. Friedländer's windmill and its accessories were prominently installed at the north entrance to the main exhibition hall ("Rotunde") in the Vienna Prater.

In July 1887, Scottish academic James Blyth installed a battery-charging machine to light his holiday home in Marykirk, Scotland. Some months later, American inventor Charles F. Brush was able to build the first automatically operated wind turbine after consulting local University professors and his colleagues Jacob S. Gibbs and Brinsley Coleberd and successfully getting the blueprints peer-reviewed for electricity production. Although Blyth's turbine was considered uneconomical in the United Kingdom, electricity generation by wind turbines was more cost effective in countries with widely scattered populations.

In Denmark by 1900, there were about 2500 windmills for mechanical loads such as pumps and mills, producing an estimated combined peak power of about 30 megawatts (MW). The largest machines were on 24-metre (79 ft) towers with four-bladed 23-metre (75 ft) diameter rotors. By 1908, there were 72 wind-driven electric generators operating in the United States from 5 kilowatts (kW) to 25 kW. Around the time of World War I, American windmill makers were producing 100,000 farm windmills each year, mostly for water-pumping.