Abstract: The invention relates generally to passive infrared markers and especially to markers which are reflective in the thermal and/or near infrared wavebands. A thermal infrared marker device is provided taking the form of a truncated pyramid, preferably a hollow truncated pyramid, said device comprising n side facets, where n?3, and a top facet (13), wherein two or more side facets comprise a thermally reflective material (17). The device may take the form of a foldable blank comprising mutually engageable side facets (21), optional fold lines (14) and mutual attachment means (15). The invention has particular utility in military applications.