We describe the ESR spectrometer we developed. Our aim was twofold: i) to reach the highest possible frequency and ii) to devise a frequency tunable spectrometer. The tunable source is an optically pumped far infrared laser which has been used from 160 GHz up to 525 GHz with magnetic fields of up to 19 T. We present measurements performed in semiconductor physics and in molecular chemical physics. These measurements allowed us to distinguish electric dipolar transitions from magnetic dipolar transitions. The increase ing-factor resolution was used to discriminate between entities withg-factors differing by a few 10−5. This property together with the study of the line-width frequency dependence was used in geophysics. We studied the spin relaxation mechanisms of the model system Phosphorus doped Silicon. The variation of the spin relaxation time with temperature shows the importance of two-phonon mechanisms. High frequency tunable ESR makes possible the study of compounds with large zero field splitting which are ESR silent at standard frequencies.