ABSTRACT:In recent years, electron paramagnetic resonance (EPR) has been extended to high magnetic fields requiring superconducting magnets, by analogy with similar extensions in NMR some decades earlier. One major difference between high-field EPR and NMR is that the corresponding increase in applied resonant frequency has required a significantly different technology in the case of EPR. In contrast to the waveguide-based systems used in conventional EPR fields, high-field EPR has required quasioptical technology that is more commonly used in radioastronomy and communications. This article presents an overview of quasioptical methods as they have been applied in high-field EPR systems. Two matrix methods for designing and engineering quasioptical circuits are presented, and their application illustrated with practical examples. Methods for evaluating the performance of the magnet and sensitivity of the quasioptics are also surveyed.