This new edition describes the state of the art regarding molecular catalysts such as MN4 metal complexes, like porphyrins and phthalocyanines. This volume focuses on the particular case of the electrocatalysis of the reduction of O2 for practical applications in fuel cells and air batteries. Indeed, active and stable materials have been developed in the last 5 years where MN4 catalytic systems can be obtained by the pyrolysis of starting materials that do not necessarily involve MN4 complexes. These latter systems constitute a new class of stable and highly active non-precious metal catalysts for ORR that can replace expensive Platinum containing electrodes. The book also offers future projections and points out new fields of research and development of these non-precious metal catalysts.