Abstract: A method and apparatus for passive identification includes a series of elements preferably in the form of strips, bars or other geometric shapes, or as dipole antennas made of a material having variable EM retro-reflectivity characteristics. By varying the characteristics of individual elements (i.e. spatial, polarization, frequency response and angular extent of retro-reflection), the elements form an alterable pattern which can be used for identification. The characteristics can be changed remotely, the ability of the device to understand and implement the change being a criterion for identification.