An optically detectable security marker for emitting light at a pre-selected wavelength. The marker comprises a rare earth dopant and a carrier incorporating the rare earth dopant. The interaction of the carrier and the dopant is such that the fluorescent fingerprint of the marker is different from that of the rare earth dopant. The marker may be incorporated into a plurality of items, such as fluids, for example paint, fuel or ink, and laminar products such as paper or banknotes or credit cards.