​The application of terahertz spectroscopy to studying electron spin resonance in high-spin molecular complexes is a new frontier. The high time resolution of single-cycle THz pulses and the broad bandwidth makes for a fresh take on traditional electron paramagnetic resonance (EPR) techniques. For example, intramolecular spin-orbit interactions lead to zero-field splittings of magnetic sublevels in the 0.2-10 THz range in certain classes of transition metal and rare-earth complexes. Few laboratory methods exist that can both reach these frequencies and span the frequency range of transitions in these complexes. Furthermore, no technique to date has been able to measure short time-scale dynamics of these systems out of equilibrium (e.g. following photoexcitation). We have developed an experimental setup that allows us to conduct a variety of THz spectroscopy experiments under high magnetic field (up to 9T). We have performed linear THz measurements to measure THz-frequency zero-field splittings in transition metal complexes. We have also studied a variety of condensed phase systems including multiferroics, magnetoacoustic thinfilms, and THz microcavities using this technique.