In the field of energy conversion devices operating under hydrogen and oxygen like fuel cells, the oxygen reduction reaction (ORR) is of primary importance for the future energy transition. However, ORR necessarily requires an electrocatalyst to be exploited. At the state of the art, platinum group metal-free (PGM-free) electrocatalysts based on carbonaceous nanostructures, heteroatoms, and coordinated transition metals are the most promising alternative to carbon-supported platinum nanoparticles (Pt/C), which are more expensive and more performing, but highly prone to deactivation in contaminated working environments. The comparison of the two materials is at the level of fine-tuning requiring specific activity descriptors, namely turnover frequency and site density, to understand how to improve the performance of PGM-free over Pt/C. Regarding the site density, specific probing molecules that bind with the active sites are required to evaluate the number of active sites of PGM-free electrocatalysts. However, PGM-free sites do not possess a single type of active site like in the case of Pt/C, but rather a multitude of primary (metal-containing) and secondary (metal-free) sites arising from the pyrolysis synthesis process, eventually complicating site density evaluation.