Distribution networks are only partially monitored networks, with loads often unbalanced on the phases of the system. Moreover, last generation of distribution networks are facing significant challenges, due to, as an example, the increasing amount of distributed generation (DG), installed in an unplanned manner. Such changing scenario emphasizes issues such as voltage unbalance and imposes new operational requirements, such as distributed voltage control and demand side management. Traditional state estimation techniques are a reliable instrument for monitoring the transmission system, but their application to distribution systems is still under development. In this work, a distribution system state estimator (DSSE) suitable for accurately monitoring unbalanced distribution networks is presented and the impact of the different kind of measurement devices is analyzed and discussed by means of simulations performed on the 13-bus IEEE distribution network.