Single-mode high-Q resonators are broadly employed in X-band (9-10 GHz) EPR spectrometers for the best possible concentration sensitivity when operating in continuous wave (CW) mode while loop-gap and dielectric ring structures are preferred for pulse EPR to generate the highest B1 field on the sample. Unfortunately, the use of these structures at mm-wave frequencies above 90 GHz becomes problematic as linear dimensions scale down to less than a few mm and non-resonant losses increase with frequency. Recently, we demonstrated a radically new solution to this long-standing problem by developing mm-wave resonators based on one-dimensional photonic band gap (PBG) crystals assembled from λ/4 low-loss dielectric layers with alternating dielectric constants. Here we report on the recent progress in developing PBG resonators with Q-factors >300 at 94 GHz by optimizing geometry of the incoming mm-wave beam and increasing sample diameter from 12 to 36 mm. More importantly, the use of low loss /high-ε materials improved the resonator finesse values up to ca. 50% of the Q-factor. Conversion factors of these resonators were evaluated in nutation experiments using 100 μm thick polystyrene film doped with 1 mM of BDPA. While the shortest 90o pulses for the best PBG were still 50% longer vs. those achieved with cylindrical TE012-type cavity of comparable Q (34 ns vs. 23 ns, respectively) when using only 0.6 W of incident power, an increase in sample volume from 0.8 to 120 μl resulted in >60-fold signal gain for the same spin concentration. A similar design was successfully employed for DNP at 200 GHz/300 MHz frequencies. PBG resonators described here are readily scalable to higher resonance frequencies and suitable for EPR and DNP of broad range of systems – from aqueous biological sample to thin film spintronic materials. Supported by NIH R01GM130821.