The chapter aims to measure the return difference with respect to the forward short circuit transfer admittance. It shows that the return difference can be evaluated by measuring two driving-point impedances at a convenient port in the feedback amplifier. The zeros of the network determinant are called the natural frequencies. Their locations in the complex frequency plane are extremely important in that they determine the stability of the network. For a physical network there remains the difficulty of getting an accurate formulation of the network determinant itself, because every equivalent network is, to a greater or lesser extent, an idealization of the physical reality. As frequency is increased, parasitic effects of the physical elements must be taken into account. What is really needed is some kind of experimental verification that the network is stable and will remain so under certain prescribed conditions. The measurement of the return difference provides an elegant solution to this problem.