The engineering aspects of system planning require various objectives to be simultaneously accomplished in order to achieve the optimality of the power system development and operation. These objectives usually contradict each other and cannot be handled by conventional single optimization techniques, while multi-objective (MO) methods fit naturally. This is the case of the placement of a large amount of distributed generation (DG) into an existing distribution network, that has simultaneously the potentiality of achieving great savings (e.g. deferment of investments and reduction of power losses) or causing technical problems (e.g. overvoltages and/or overloads), depending on the DG size and location. In this paper, an evolutionary MO procedure for the optimal DG siting and sizing is proposed. The goal of this methodology is to establish the best penetration level of DG, which maximizes the benefits of the presence of generators in a distribution network, as well to limit the network performance deterioration due to DG not connected in optimal locations. An application example is presented to demonstrate the effectiveness of the proposed procedure.