The development of compound semiconductor, wide bandgap electronics based on GaN is poised to impact future rf and advanced power electronic switches. The performance and reliability of these devices is impacted by heat dissipation, which is necessary in order to maintain reasonable device temperatures. Diamond is considered to be a suitable material to promote heat transfer away from the active regions of devices, and recently, several techniques have been developed to integrate diamond with semiconductor materials to achieve effective heat dissipation. We address the properties of polycrystalline diamond, both in thin film and bulk form, to better understand how the polycrystalline nature of diamond impacts thermal conductivity. The deposition of diamond onto, for example, GaN requires the inclusion of interfacial layers; the stability of these layers plays a large role in their effectiveness to protect the semiconductor from the deposition byproducts and to facilitate heat transfer from the GaN to the diamond layer. Novel surface engineering techniques also impact thermal transport across interfaces. We employed transmission electron microscopy imaging and electron energy loss spectroscopy, electron backscattered diffraction (SEM) and precession electron diffraction(TEM), high resolution x-ray scattering, combined with thermal transport measurements to address these issues.