Display omitted •Synchrotron based frequency-domain Fourier-transform THz-EPR (FD-FT THz-EPR).•Assign spin-coupling parameters of high-spin systems with large zero-field splittings.•Instrumental foundations and technical peculiarities of FD-FT THz-EPR.•Selected applications of FD-FT THz-EPR. We describe frequency-domain Fourier-transform THz-EPR as a method to assign spin-coupling parameters of high-spin (S>1/2) systems with very large zero-field splittings. The instrumental foundations of synchrotron-based FD-FT THz-EPR are presented, alongside with a discussion of frequency-domain EPR simulation routines. The capabilities of this approach is demonstrated for selected mono- and multinuclear HS systems. Finally, we discuss remaining challenges and give an outlook on the future prospects of the technique.