摘要： Since about 35 years ago, the development of the electron microscopy and spectroscopy techniques has pushed the scienti?c community toward the fabrication of physical systems with a reduced “vertical” dimension, down to nanometer size and ultimately to the monolayer (ML) or single-atom thickness. Especially at the beginning, a requirement for these ultra-thin systems was that these should be grown on metallic or at least semiconducting substrates in order to exploit the enhanced resolution of surface science techniques available when working with conductive systems. Oxide systems were no exception to this trend, and systems made by one or very few oxide layers grown onto metallic substrates were indeed achieved and started to be thoroughly characterized.1,2 Another push to research on oxide ultra-thin ?lms was provided by semiconductor device technology, in which silicon chips are covered by a nanometer-size ?lm of amorphous silica, by the efforts in understanding metal corrosion, by the study of thin insulating oxide ?lms in storage devices, and by the study of oxide supports in heterogeneous catalysis. The ?eld however evolved independently of these initial drives especially as it became clear that the properties of these materials, such as transport and electronic structure, were strongly in?uenced by their detailed and unusual structure. Quite soon, in fact, it was realized that these systems in most cases were not a mere reproduction of the bulk oxides on a reduced scale, but represented completely new hybrid metal/oxide systems with novel structural and electronic properties, often with no counterpart in the bulk. This was unequivocally demonstrated when the atomistic structure of some of these phases was unveiled via the combined use of microscopy and computational characterization,3–5 which allowed researchers to investigate not only the novel structures thus produced, but also the corresponding electronic and spectroscopic properties in great detail. It was the birth of the ?eld of UltraThin Oxides (UTOx) on metal surfaces, whose theoretical or electronic structure description is the topic of this essay.