Magnetorheological (MR) fluids, which can rapidly change from a liquid state to a solid state and vice versa in a magnetic field, have potential to revolutionize several industrial sectors. The key issue is to enhance the yield shear stress of MR fluids. This chapter reviews the physical mechanism and microstructure of MR fluids. It finds that the weak points of the MR microstructure under a shear force are at the chains' ends. Hence, a general technique, compression-assisted-aggregation process, is developed to change the induced MR structure to a structure that consists of robust thick columns with strong ends. The scanning electronic micrographic images confirm such structure change. With this approach, MR fluids become super-strong. The enhanced yield stress of MR fluids reaches 800 kPa at a moderate magnetic field.