Due to a large variety of properties, passive films and their stability play an important role in recent and future research. At first, the nature of passive films is discussed for the steady state. New techniques allow the detailed determination of film thickness, stoichiometry, microstructure, and electronic properties. The distinction of these properties allows a rough classification of oxide films and their properties. In the second part, various processes, such as growth, reduction, dissolution, and modification are discussed. These overall processes depend on the rate of ion and electron transfer reactions (ITR, ETR) in a complex manner. Examples are given for some special reactions. Applications are discussed for surface protection, micro and nano systems and electrocatalysis.