Abstract: Method and means are disclosed for detection of overlapped sheets transported along a path. A plurality of detection means are aligned substantially perpendicular to the direction of movement of said sheets, each detection means including one or more radiation sources and one or more radiation detectors. The overlapping of one sheet by another produces a shadow on the surface of the overlapped sheet along the edge of the overlapping sheet when radiation is directed obliquely on to the sheets from one of said radiation sources, the direction of such radiation necessary to produce a shadow being dependent on the direction of the overlap of one sheet by another. Reflected radiation from the sheets is received by one or more of the radiation detectors, and shadows or marks on the sheets are detected by reason of their lower reflectivity.