A secure and/or valuable document, such as a banknote (1), incorporates verification means (11;21;31;41) for verifying a security feature (10;22;32;43) on the same or a different security document when the verification means and the security feature are laid over one another. Examples of verification means comprise optical lenses (11), graphic elements, optical filters (21;31), interference filters or line or dot grids (41). An optical filter may take the form of a colour tinted filter (21) for viewing a security element (22) comprising area printed with metameric inks on the same or a different security document. Another form of optical filter is a first transparent polarising window (31) for verifying a second transparent polarising window (32) on the same or a different security document.