Magnetorheological fluid (MRF) is a new type of smart material, primarily composed of magnetic particles, a base liquid, and a stabilizer. Under the influence of an external magnetic field, the magnetic particles attract each other and quickly form chain-like or sheet-like microstructures, which restrict the flow of the matrix. As a result, the shear stress increases by two to three orders of magnitude in a very short time, a phenomenon known as the magnetorheological (MR) effect. Due to the significant changes in mechanical properties, rapid response, and the ability to return to its initial state after the magnetic field is removed, MRFs have been widely used in industrial fields such as equipment vibration reduction, building seismic resistance, and more. This chapter provides a detailed introduction to MRFs, covering their preparation, magnetorheological and magneto-mechano coupling properties, formation mechanism, and applications.