Fuel　cells　are　regarded　as　one　of　the　most　promising　candidates　for　stationary　and　mobile　power　generation　due　to　their　high　energy　yield　and　low　environmental　impact　of　hydrogen　oxidation.　The　oxygen　reduction　reaction　(ORR)　at　cathode　is　a　very　complex　process　and　plays　a　crucial　role　during　operation　of　the　PEM　fuel　cells.　However,　its　mechanism　and　the　nature　of　intermediates　involved　remain　vague.　This　chapter　focuses　on　the　recent　theoretical　modeling　studies　of　ORR　catalysts　for　PEMFC.　Recent　theoretical　investigations　on　oxygen　reduction　electrocatalysts,　such　as　Pt-based　catalysts,　non-Pt　metal　catalysts　(Pd,　Ir,　CuCl),　and　non-precious　metal　catalysts　(transitional　metal　macrocyclic　complexes,　conductive　polymer　materials,　and　carbon-based　materials),　are　reviewed.　The　oxygen　reduction　mechanisms　catalyzed　by　these　catalysts　are　discussed　based　on　the　results.　©　Springer-Verlag　London　2013.