The understanding of metal passivation has greatly improved in the recent years due to the development of in situ experimental techniques. It is now possible to characterize at the nanoscale the chemical composition, structure and electronic properties of oxide films that grow on the surface of metals, while keeping them in solution and under potentiostatic control. This knowledge is allowing to elucidate the mechanisms of metal passivation and is providing great advances in the characterization of other dynamic, complex processes like passivity breakdown induced by chloride.