The phenomenon of electrorheology is discussed in relation to possible mechanisms involving polarization of dispersions in flow. Data relating such polarization during flow are presented, and comparisons between electrorheological activity and dielectric properties are made. The importance of the rate of polarization as well as its magnitude is stressed. How such considerations led to electrorheological fluids based on dispersed semiconducting polymers as an alternative to traditional water containing polyelectrolytes is discussed, and some of the dielectric and electrorheological properties of these new electrorheological fluids are described. © 1990, American Chemical Society. All rights reserved.