The microscopic origin of temperature induced transition frommetal-to-insulators are studied. The question is addressed, how and wheredoes the carriers (electrons) freeze out, when the material is gettinginsulating. Materials to be studied are mainly transition metal oxides,materials related to perovskites of type ABO3. To study this effect indetail, resonant X-ray scattering based on synchrotron radiation is usedat the newly built Swiss Light Source at the Paul Scherrer Institute. Thistechnique allows us to study ordering phenomena of electrons and orbitals(aspheric electron distribution around the individual ions), which areoften occurring together with the order of magnetic moments at themetal-to-insulator transition. These studies are combined with studies onthe crystalline structure of these oxides, which is usually also affectedby such transitions. The fundamental understanding of the origin of thesephase transitions, triggered by electronic ordering phenomena, isimportant by itself, but does have impact for the understanding ofhigh-temperature superconductivity and the effect of colossal-magnetoresistance.