Abstract This paper deals with the application and optimization of active loops for the mitigation of the magnetic field generated by complex sources, especially MV/LV substations. Various active-loop configurations are proposed, which are analyzed in several regions located close to a particular MV/LV substation, and the optimum geometry and location of the loops are obtained together with the optimum current (magnitude and phase) to be injected into the loops. All this is performed by means of a genetic algorithm in order to reduce the average magnetic field inside the target volume (inspection volume) to the minimum. The performance of the proposed active loops is compared with that obtained via other well-known mitigation technique, such as conductive/ferromagnetic passive shields. The main results are analyzed, and the differences, in terms of performance, between the two shielding techniques, and their proper application to MV/LV substations, are highlighted.