An electrorheological (ER) suspension is made from an insulating liquid medium embodying either a semi-conductive particulate material or a semi-conductive liquid material (usually a liquid crystal material). Since its mechanical properties can be easily controlled over a wide range (almost from a pure liquid to a solid), the ER fluid can be used as an electric and mechanical interface in various industrial areas, for example, in the automotive industrial for clutch, brake and damping systems and in robotic arm joints and hands. In addition, the ER technique can be used to fabricate advanced functional materials such as photonic crystals, smart inks, and heterogeneous polymer composites. The major objective of Electrorheological Fluids is to present a comprehensive survey on the ER suspensions in term of screening high performance ER materials, physical mechanisms of the ER effect, and the applications of ER technology.