An identification tag has at least one molecule of a first type and at least one molecule of a second type. The molecules are capable of energy transfer according to Förster Resonance Energy Transfer, with one of the types being donor molecules and the other type being acceptor molecules. The tag has a substrate which has a plurality of locations at which molecules of the second type may be located. The light intensity emitted by the acceptor molecule(s) as the molecule of the first type is moved across the locations varies in dependence on whether a molecule of the second type is or is not located in the locations.