Increasing the number of measurements in power distribution networks is crucial in improving the monitoring accuracy and operation quality. However, due to the large number of nodes in distribution networks, it is not economically possible to install meters in all nodes. State estimation as a useful tool can help to reduce the need for meters. The location of meters has a significant effect on the state estimation results. A comprehensive literature review of the optimal meter placement methods in distribution networks with the aim of improving state estimation is presented in this paper. These methods are compared in terms of the objective function and constraints of the optimization problem, relevant uncertainties, types of state estimation algorithms and the solution methods. This paper provides a detailed insight for researchers and network operators about the types of meter placement methods, their advantages and disadvantages, existing barriers, and future research areas.