State estimation operation depends on the measurement numbers, types and locations. Therefore, the adoption of an efficient placement method for selecting the required numbers of meters and their location is an essential process for power systems monitoring. In fact, the Phasor measurement Unit (PMU) is, currently, one of the most efficient devices for measuring real-time electrical quantities. Consequently, this paper aims to present the main categories for PMUs placement strategies according to the objectives of PMUs employment. The PMUs placement strategy should take into consideration main factors that required for enhancing the performance of a state estimation process such as reliability, accuracy and the special requirements of the modern distribution grid. A review of literatures that address PMUs placement methods is presented in this paper in addition to suggestions for developing the PMUs placement methods to be more feasible for modern power networks.