Thermoelectric generators (TEGs) convert heat into electricity, and could contribute to the world's increasing energy demand by harvesting low-energy-density heat, such as waste heat produced during the conversion of fossil fuels to electricity or heat from solar radiation. The efficiency of the heat-electricity conversion is dictated by the material properties. This chapter presents the key material properties that help define the heat-to-electricity conversion efficiency. TEG is an electronic device that uses the Seebeck effect to convert a heat flow into an electron flow. The Seebeck coefficient is intimately related to the electronic structure and mobility of the charge carrier. Controlling the shape of the density of state at the Fermi level in a material should enable tuning its Seebeck coefficient. Hence, it is crucial to understand the electronic structure of conducting polymers. The chapter summarizes how the electrical conductivity, the Seebeck coefficient, and the thermal conductivity of conducting polymers depend on their oxidation level.