Metal clusters deposited on oxide surfaces exhibit specific chemical and physical properties often connected to their low dimensionality. The properties of truly nano-clusters a few atoms in size or even of isolated metal atoms are highly depending on the type of oxide support, adsorption site, surface morphology, etc. One of the aspects that can deeply modify the properties of the adsorbed metal atoms or clusters is the occurrence of charge transfers at the metal-oxide interface. Recent studies have shown that charged clusters, and in particular cluster anions, are catalytically more active than their neutral counterparts. Charging mechanisms of metal atoms and clusters on oxide surfaces is the topic of this talk. Using high quality electronic structure calculations in combination with experimental spectroscopic measurements, we will discuss in which conditions charging occurs and how to measure it. The systems considered are metal atoms like Cu, Au, Pd, on oxide substrates like MgO, SiO2, and TiO2 single crystals or in polycrystalline or amorphous form. We will show that a major role in charging of the deposited atoms is played by point defects at the oxide surface and discuss methods to prove the occurrence of the charge transfer. In the second part of the talk we will examine methods to induce charging even without implying the presence of defects. In particular, we will consider metal atoms deposited on ultra-thin oxide films epitaxially grown on metal single crystals. We will discuss the adsorption properties of Pd, Ag, and Au atoms on 1 to 5 layers thick films of MgO on Mo(100) and compare them to those of MgO(100) single crystals. On supported MgO thin films charging can occur from the metal substrate to adsorbed atoms with high electron affinities, like Au. We will discuss possible mechanisms for this charge transfer like direct tunneling or dielectric breakdown induced by an external electric field.