Since the lower and higher cut-off frequencies of an amplifier depend on poles and zeros and since the negative feedback enlarges the bandwidth, it can be concluded that the feedback moves poles and zeros on the Gauss complex plane. In principle, it has thus to be expected that the feedback might move a pole from the negative to the positive half plane, making the amplifier unstable. Against advantages as reduced non-linearities, reduced sensitivity to the parametric variations, enlarged bandwidth, and better bias point stability, the feedback thus implies as drawback, in addition to the gain reduction, also the more insidious potential instability. This chapter is devoted to the rules useful for investigating the asymptotic stability of a feedback amplifier and the methods may be adopted to make it stable.