The second session on materials began with a talk by Thumm, et al. (University of Karlsruhe and Forschungszentrum Karlsruhe, Germany) on the current state of materials used in K-band devices. Thumm first reviewed the advantages materials can provide millimeter-wave device developers. He discussed the physical and technological advantages of producing millimeter-wave versus microwave devices. Thumm finished with a comparison of efficiencies and power versus size, weight and accelerating voltages for a 24.15 GHz magnetron, klystron, extended interaction oscillator and gyrotron. Next Choi et al (Kwangwoon University, Korea) presented data on recent dielectric measurements of beryllia, Teflon and CVD diamond using two Fabry-Perot open resonators developed for low-loss materials in Ka-band (26-40 GHz), V-band (50-75 GHz) and W-band (75-110 GHz). Gamble et al. (DSTL, MAAS and DeBeers, England) discussed the use of CVD diamond as a waveguide window in a high-powered, wide-bandwidth, free-electron maser. Gamble described the loss requirements the window must maintain under CW operation and the predicted window thickness from HFSS modeling. Finally, he presented experimental results on CVD diamond window, built to the model thickness and bonded into a waveguide. Both insertion and transmission losses measured were in agreement with model predictions. The final talk of the session was presented by Bosman et al. (University of Michigan, USA) describing a theoretical model of diamond window failure. The model evaluated proposed that the failure mechanism may be based on formation of graphite from amorphous carbon at grain boundaries. Stresses induced by increasing graphite formation would then be the cause of destructive failure. However Bosman, et al.’s results showed that the temperatures required to cause the reaction leading to the failure are thermodynamically improbable and that a new model should be sought.