The objective of this dissertation is to advance the fundamental understanding of the role of crystal defects on thermoelectric (TE) properties of wide band-gap materials in order to enable a new class of TE material for high temperature power generation. Wide bandgap semiconductors, such as GaN and ZnO based materials, are promising candidates for high temperature TE applications due to their superior mechanical performance and thermal stability at high temperatures. Both are currently integrated in many commercial applications including solid state lightning, power transistors, and solar cell applications. However, their inherent high thermal conductivity must be reduced, without compromising electrical properties, in order to achieve a high TE efficiency. In this study, the impact of intrinsic defects (e.g., point, line, etc.) and extrinsic defects (e.g., dislocations, cracking, etc.) on the TE properties of these wide band gap materials are investigated. Advanced characterization techniques are used to investigate the structural, electrical, and thermal properties of these materials (and a detailed modeling process and theoretical work are explained). Additionally, an examination of induced extrinsic defects used to tune the TE properties of materials (which includes minimizing or maximizing certain intrinsic defects, adding point defects by doping, and investigating different alloys) is performed. In this research, the bulk and thin film of GaN and ZnO, and their ternary alloys, such as InGaN are focused. The low dimensional structures, AlGaN/GaN superlattices are also investigated. This study further introduces a comprehensive literature review of TE properties of various materials in two categories: common bulk TE materials and new TE materials. An in-depth theoretical investigation of electron and phonon transport mechanisms using Boltzmann transport equation and Callaway approximation, respectively are included in this study. This research will finally produce a set of proof-of-concept nitride and oxide samples for high temperature TE devices. Metalorganic chemical vapor deposition (MOCVD) is used for material processing to enable cost effective, highly thermally stable, and nontoxic TE materials for waste heat recovery at high temperatures (>1000K). The knowledge gained from this study will advance the understanding of the relationship between the structural, electrical, and TE properties of wide-band gap materials, GaN and ZnO. Such fundamental knowledge will result in novel high-performance TE materials and contribute in designing TE generators based on these materials; moreover, it could provide a solution for high-temperature TE generators with abundant, cost-effective, and environmentally benign materials for monolithic integration to next-generation solar thermal devices.