The effect of time‐varying electric fields on the stresses in model and commercial electrorheological (ER) fluids in steady shear were surveyed. The dependence of the time‐averaged shear stress with the frequency of an applied alternating (ac) square‐wave electric field reveals two classes of fluids: those that exhibit substantial ER shear stresses with static (dc) and low frequency ac fields, and those that perform poorly in dc fields but show substantial ER activity in ac fields. These contrasting behaviors are consistent with the existence of two different means by which ER activity is generally produced: mismatches of the conductivities and dielectric constants, respectively, between the suspended particles and suspending fluids. Sudden changes in the applied electric field were found to induce a variety of transient stress phenomena; while the detailed response of the disparate materials studied did differ, many similarities were observed. The characteristic times for the growth and decay of the stress were found to depend inversely on the shear rate; they varied only weakly with the amplitude of the applied field. Rise and decay times as short as a few ms were observed at the highest shear rates utilized. The inverse relationship between the characteristic rheological response times and the shear rate is consistent with the necessary roles of both field‐induced structure formation and shear‐induced strain growth in producing an ER shear stress. Implications of this work include that high shear rates must be designed into ER devices in order to achieve short response times.