A prosthetic leg is a device designed to replace, as much as possible, the function or appearance of a missing limb. Vibrations caused during walking or running of a human is an important factor to be considered while designing a prosthetic leg. This book focuses on the use of Magneto-Rheological fluid to control these vibrations. When the fluid is contained between two poles, the resulting chains of particles restrict the movement of the fluid, perpendicular to the direction of flux, effectively increasing its viscosity. A variety of proprietary additives similar to those found in commercial lubricants are commonly added to discourage gravitational settling and promote particle suspension, enhance lubrication, modify viscosity, and inhibit wear. The advantage of using MRF in the design of the actuator in comparison with a conventional damper is that the damping coefficient of the actuator is more.