Electron paramagnetic resonance at high magnetic fields (up to 25 T) and high frequencies (sub-THz: 95 – 750 GHz), known as HFEPR, has been developed over the past two decades. There are many applications of HFEPR, such as in the precise determination of g values in systems with multiple radical species (e.g., photosynthesis). Another application is in transition metal coordination chemistry: there are many paramagnetic complexes for which conventional EPR (fields below 1.5 T, frequencies up to 35 GHz) is less than ideal. Primarily, such systems are high-spin, wherein the effects of zero-field splitting can make the complex either “EPR-silent” using conventional EPR, or the EPR spectrum is not particularly informative, such as for the “g = 4.3” signal in Fe(III). We will describe the use of HFEPR to study high spin first row (3d) transition metal complexes (from Group 4 through Group 10) with both “classical scorpionate” ligands (i.e., trispyrazolylborates) and “non-classical scorpionates” (i.e., trisimidazolylborates), in conjunction with other techniques such as MCD and Mӧssbauer. We will also discuss how theory can be used to provide chemical information, based on analysis of the spin Hamiltonian parameters determined by HFEPR.