This chapter looks at the renewable energy technologies that are reshaping the energy landscape: wind power and photovoltaic electricity. It shows how wind turbines are becoming larger and more efficient, and how huge arrays of photovoltaic panels are now providing least-cost large-scale power in countries around the world. An overview of the principles used to design large wind turbines and photovoltaic power systems is presented. Other renewable energy technologies also show promise: particularly concentrating solar power, biofuels, and geothermal energy. Hydropower, which has been a mainstay of power systems in many countries for decades, continues to make a substantial contribution but is not expected to greatly increase its capacity in North America and Europe. Biomass sources of energy include wood-fired power plants and biofuels—both of which will play an important role in phasing out coal and replacing gasoline and diesel fuel.