Fuel cells and hydrogen generators are important electrochemical energy conversion devices for future energy technology. Fuel cells convert pure hydrogen or other hydrogen rich fuel into DC electricity. In a hydrogen generator, the reaction is reversed and water is electrochemically split into hydrogen and oxygen. During the last decade, polymer electrolyte membrane (PEM) based components have played a key role in both fuel cell and hydrogen generator development. The major conversion loss in a PEM fuel cell takes place on the oxygen reduction cathode and in a PEM hydrogen generator on the oxygen evolution anode. Molecular modeling has been combined with experimental work in order to optimize nanostructured catalysts for both types of electrodes.