Penetration of distributed generation (DG) units in distribution system has increased rapidly stimulated by reduced network power loss, improved bus voltage profile and better power quality. Appropriate location and capacity of DG play significant role in maximizing beneficial effects. The aim of the optimal DG placement (ODGP) is to provide the best location and capacity of DGs to optimize electrical distribution network operation and planning taking into account DG capacity constraints. Different methods applied to the OPDG problem are reviewed in this paper.Future research trends in this field are also analyzed.