The optical switching time of twisted-nematic liquid-crystal cells using the liquid crystals, 5CB (C,H,,-Ph-Ph-CN), 50CB(C,Hw O-Ph-Ph-CN) and PCH5 (C,H,,-Cy-Ph-CN) have been characterized as a function of temperature, prebias voltage and switching voltage, V. The transition time from 90 % to 10 % transmission scales as V-1.9 and is limited to 30 to 50 ns by the liquid-crystal breakdown electric field, - 100 V I'm-I The time fi-om the initial switching voltage step to 90 % transmission, delay time, decreases with increasing prebias and switching voltage. For 5CB and 50CS the delay time approaches a constant value at higher electric fields, >10 V ~1Il,-1. Both the transition and delay times decrease with increasing temperature. The minimum transition time at temperatures a few degrees below the nematicisotropic temperature are 32, 32, and 44 ns and delay times are 44, 25 and 8 ns for 5CB, 50CB, and PCH5 respectively.