Distributed energy resources, or distributed generation (DG), are the mainstay of modern power systems aiming towards green energy generation via the effective integration of renewable energy sources. DG involvement in traditional power systems comprises the enhancement of power quality parameters, the utilization of renewable sources, cost optimization, and stable and reliable energy generation. The advantages of such a revolutionary approach can be achieved with the optimal sizing and allocation of DG by means of adequate optimization techniques, constraints, and optimized parameters. In this study, a comprehensive review of DG optimization is presented in light of recent developments, and a comparison is carried out based on the adopted optimization techniques, test system, enhanced parameters, and outcome evaluations.