Middle ear hearing reconstruction is unpredictable. Difficulties arise because of host factors, such as ventilation or scarring, surgical technique factors, such as prosthesis placement and stabilization, and design and mechanical factors influencing the properties of the prosthesis. Often there is a balancing act between choosing optimal stability, and maximizing the mechanical vibrations of the prosthesis. We review our and other investigators' work, in design and ideal placement of middle ear prostheses. Middle ear prostheses need to be rigid enough to deliver acoustic forces without bending. Prosthesis mass has a modest effect at higher frequencies. A key point is that rotational movements of the prosthesis have to be constrained. Prosthesis head size and cartilage interposition, within reason, have little effect on vibration transmission. Reconstruction to the malleus may have some small mechanical advantage; however this is not clearly proven. Similarly, there is no proven advantage in reconstructing to the stapes head instead of the footplate. The most important factor for good long term results is probably the stability of the prosthesis, both to acute inertial forces such as trauma, and to slower term changes such as tympanic membrane position and scarring contractures.