Nowadays the consumption and need for the power is more hence there is great demand on power consumption by the people which may often cause instability in power system. This instability in Voltage or current in the power system can be controlled using various kinds of facts device. Hence, this paper explains the role of the facts devices in maintenance of stability and their optimal placements in power system. Thus this paper gives a comprehensive and comparative study of different types of facts device and their role in Voltage or transients stability analysis methods.