The aim of this project is to further improve the sensitivity of high frequency electron paramagnetic resonance (HFEPR) techniques. The improvements will enable frequency domain measurements on samples with small numbers of spins, allowing investigation of real-life samples such as metalloenzymes, (nanostructured) thin layers and heterogeneous catalyst nanoparticles. We will build on the achievements from the first funding period, where we were able to improve the sensitivity of terahertz frequency domain magnetic resonance spectroscopy by three orders of magnitude. This was accomplished by successfully implementing a field-modulation scheme, as well as by largely removing standing waves from the terahertz beam. In the continuation of this project, we expect to gain a further two orders of magnitude sensitivity improvement. This will be realized by further elimination of standing waves, as well as incorporation of a broadband tunable Fabry-Pérot resonator. We aim to achieve a 106-spin-sensitivity. Secondly, we will implement terahertz rapid scan EPR both in field and frequency domains for the first time. In the frequency domain, this will speed up measurements by four orders of magnitude, compared to conventional field-swept high-frequency EPR techniques. In addition, these measurements will permit us to access the spin dynamics of the systems at terahertz frequencies, which is otherwise not easily accessible. Thirdly, we will perform measurements on real life samples such as thin layers of molecular quantum bits and molecular nanomagnets, as well as metal-oxide catalyst nanoparticles. Finally, we will implement the novel method of terahertz optically detected EPR, where the optical detection will be implemented through measurement of the magnetic circular dichroism.