<p>A power system is subject to disturbances that may lead to voltage stability problems, which eventually may lead to black-outs. On-line phasor measurements may allow anticipating situations leading to voltage instabilities; these on-line measurements can be accomplished by using Phasor Measurement Units – PMU. However, for a power system is not economical or necessary to install PMU at each bus; in order to avoid this situation, it is required to develop a strategy for PMU placement that is useful and convenient, with a limited number of available PMU. This paper presents a review and comparison of some methods for PMU placement in power systems; also, a classification according to the type of observability is done and the application of the method is considered. Finally, a method for monitoring voltage stability in power systems is chosen and it is tested on the IEEE39-bus system using Matlab-Power System Analysis Toolbox PSAT.<strong></strong></p>