On the application of electric fields, the suspensions consisting of polarizable particles dispersed in insulating oils undergo a rapid transition from Newtonian liquids to rigid solids, and this effect is referred to as electrorheology (ER). In electrified suspensions, the dipole-dipole interactions cause chains of particles in the direction parallel to the field vector. The fibrous structures are responsible for the solidlike responses of suspensions. When the dielectric constants of the particles and the oils are substantially different, each particle acquires an induced dipole in electric fields. Basically, the intrinsic mechanism of ER is a dielectric mismatch. However, in actual suspensions, the ER effect is strongly affected by the substances at the particle surface, rather than the particle material itself. The effects of surface conditions on the ER are discussed in relation to the method of formulating excellent ER fluids.