The Expiatory Chapel in Monza was built to commemorate the murder of the Italian King Umberto I of the House of Savoy on 29th July 1900. Despite the apparent lack of interest showed by the past literature about this monument, its ceilings, floorings and walls are fully covered by polychrome mosaics and precious marble slabs, result of a high specialized workforce that decorated what now looks like an authentic royal cenotaph. Researches currently under development could provide important data about the mosaic and marble cladding, one of the most luxurious examples of the stile umbertino.