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Magnetic and Magnetorheological (MR) fluids are formed by small magnetic particles suspended in a fluid such as water or oils. In the absence of an external magnetic field, these suspensions behave as a non-Newtonian fluid. When an external magnetic field is applied to these fluids, magnetic dipole moments are induced in the magnetic particles and many clusters of magnetic particles are formed along the magnetic field direction. Since magnetic and MR fluids are opaque, it is very difficult to analyze the inner structure using common optical methods. Ultrasonic propagation velocity and attenuation in magnetic and MR fluids change with the application of an external magnetic field. It seems that the formation of clustering structures influences on ultrasonic propagation properties. Therefore, the author proposed a qualitative analysis of these structures by measuring properties of ultrasonic propagation. In this review article, we summarized our previous experimental results.