The aim of my research is to develop a world-unique THz electron paramagnetic ellipsometry instrumentation (THz-EPR-E) using high magnetic fields and apply it to study electron spin resonances of defects in semiconductors. The goals are to study highly anisotropic systems to gain detailed information on local site symmetry and defects, to develop optical models, and to advance instrumentation and methods. The focus is on semiconductor materials of high contemporary interest for application in quantum technologies, such as group-lll Nitrides, group-lll oxides, and SiC.