Metal oxides (Ca3Co4O9, CaMnO3, SrTiO3, In2O3), Ti sulfides, and Mn silicides are promising thermoelectric (TE) material candidates for cascade-type modules that are usable in a temperature range of 3001200 K in air. In this paper, we review previous studies in the field of TE materials development and make recommendations for each material regarding future research. Furthermore, the R&D of TE modules composed of metal oxide materials and the prospect of their commercialization for energy harvesting is demonstrated.