Voltage instability has been a great concern for a long time in electrical power industry. A system enters a state of instability due to increase in demand, a sudden large disturbance or a change in system condition that causes a progressive and uncontrollable decline in voltage. It is therefore interest to study dynamic and static aspects of voltage stability.In this Book, Effects of induction motor load on short-term voltage stability and the effects of dynamics of slow-active devices on long- term voltage stability of a system were also investigated using PV curves and time domain analysis (MATLAB/SIMULINK). A technique to determine the critical fault clearing time to prevent stalling of induction motor load has been presented. In addition, a method to determine voltage stability index based on the complex voltages of all buses is proposed and used in estimating the maximum loading of the power system and for identifying weakest segments of a power system to prevent voltage instability/collapse by providing appropriate reactive compensations.