This document discusses stability analysis of discrete-time systems. It states that for a discrete-time system to be stable, all poles of its transfer function must lie inside the unit circle in the z-plane. A system is critically stable if a single pole is at z=1, or if a complex conjugate pole pair lies on the unit circle. Any multiple pole on or inside the unit circle renders the system unstable. Methods for analyzing stability include the Jury test and bilinear transformation with the Routh-Hurwitz criterion. The bilinear transformation maps the z-plane to the w-plane, where the unit circle corresponds to the left half-plane. Then the Routh-Hurwitz criterion can be applied.