Thermoelectric generators have been used for powering spacecrafts since the 1950s. Low material efficiency limits its application on a large scale. On page 6125, G. J. Snyder, Yanzhong Pei, and Heng Wang review strategies of engineering the electronic band structure, such as effective mass, degeneracy and band gap, which increase the material efficiency. These purely electronic strategies are independent of the approaches for low thermal conductivity, and may lead to revolutionary applications of thermoelectrics. The authors thank Nicholas A. Heinz for his help on the design of the cover image.