M-N-C electrocatalysts (where M is Fe or Co) have been investigated for mitigating the dependence on noble metals when catalyzing the oxygen reduction reaction (ORR) for fuel cell technologies in acidic or alkaline conditions. Rotating disc and rotating ring-disk electrodes measurements for Fe-N-C and CoN -C catalysts demonstrate promising performances and stability for the ORR, while the activity of main suspected active sites (M-NxCy and M@N-C) has been discussed on the basis of the known physical-chemical properties of the catalysts in acid and alkaline media. Thereupon, it is observed that atomically-dispersed Fe-NxCy sites reach the highest ORR activity in acid media when amplified by an adequate energy binding between the metallic center and the oxygenated reaction intermediates. In contrast, Fe@N-C core-shell sites reach a maximum ORR mass activity in alkaline media through a synergistic effect involving catalyst particles with metallic iron in the core and nitrogen-doped carbon in the shell.