Abstract: Just microns below the visible surface of Old Master Paintings lays a wealth of information about the artwork. Hidden layers may contain an underdrawing, underpainting or compositional alterations. Painters frequently re-used canvases and panels and therefore painted a new picture on top of an existing one. Therefor, a look through the paint layers offers a look over the painters shoulder. This is of interest in conservation issues and questions of attribution. In this contribution I will focus on the application of portable and sychrotron-based macro XRF elemental scanning- to study the substructure of historical paintings. I will focus on examples from the oeuvre of Vincent van Gogh and Rembrandt, where the object's substructures offer exciting new insights in the artworks' genesis and attribution.