Book excerpt: Magneto-sensitive soft materials are new synthetic functional materials that is normally composed of ferromagnetic or ferrimagnetic particles (size in a range from several nanometers to hundreds of micrometers), carriers (including water, organic solvent, liquids, gels, polymer and foams), surfactants and necessary additives. Being different from “hard” solid materials, “soft” means magneto-sensitive materials exist in the form of colloidal liquids, gels, and elastomers, such as magnetic fluids (MF), also called ferrofluids, magnetic liquids, magnetorheological fluids (MRF), magnetorheological gels (MRG), magnetorheological elastomers (MRE) and magnetorheological foams (MRFoam), so as to possess fluidity and magnetism simultaneously and can be easily deformed by applying external magnetic field force.