Abstract: A method for marking articles or documents by placing on a substrate (1) a multi-layer optical complex consisting of an identifying portion (2, 7) such as bar codes, a photograph or characters, and an authenticating portion consisting of a diffracting optical mark, said portions being arranged one on top of the other in such a way that they cannot be separated without destroying the authenticating portion. A protected document is also disclosed which comprises an indivisible stack consisting of a printed portion (2, 7) forming an identifier read by reflection in a first wavelength band, and coated with a lacquer (5) that is opaque in the visible spectrum and has a transmission window including said first wavelength band, as well as a transparent reflective layer (4) forming a diffracting optical mark. A marking system for carrying out the method, and a device for checking said article or document, are also disclosed.