In this chapter, we have covered various topics in thermoelectrics such as transport theory, schemes to enhance thermoelectric performance, material synthesis methods, device theory and possible applications. Thermoelectric performance has been improved by using nanostructures since the 2000s. Nanostructures alleviated the interdependency between three thermoelectric properties, i.e. the Seebeck coefficient, electrical conductivity and thermal conductivity, and they were particularly effective in reducing thermal conductivity. We also presented ways to enhance the power factor, such as the quantum confinement effect, electron filtering, band convergence and resonant levels. It is expected that combining these two technologies could lead to high performance thermoelectric materials. To realize these transport theories, various synthesis procedures have been scrutinized. Some of these efforts have been introduced in this chapter with an emphasis on the thermoelectric figure of merit dependency over crystal structures. Finally, device theory and fabrication were presented. We hope that this book chapter can significantly help scholars working in the field of thermoelectrics.