A wind turbine is a device that converts the energy of wind into electrical energy. As of 2020, hundreds of thousands of large turbines, in installations known as wind farms, were generating over 650 gigawatts (GW) 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.

Turbines used in wind farms for commercial production of electric power are usually three-bladed. Large horizontal-axis wind turbines with the blades upwind of the tower produce the overwhelming majority of wind power in the world today. These turbines have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind.

Generally, efficiency increases along with turbine blade lengths. The blades must be stiff, strong, durable, light and resistant to fatigue. Materials with these properties include composites such as polyester and epoxy, while glass fiber and carbon fiber have been used for the reinforcing. As of 2021, the longest blade was 115.5 m (379 ft), producing 15 megawatts (MW) of electricity.

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