PCT No. PCT / GB96 / 00823 Sec. 371 Date Jan. 12, 1998 Sec. 102(e) Date Jan. 12, 1998 PCT Filed Apr. 3, 1996 PCT Pub. No. WO96 / 31790 PCT Pub. Date Oct. 10, 1996Magnetic tags or markers are disclosed, together with a variety of techniques by means of which such tags may be interrogated. In one aspect, the magnetic marker or tag which is characterized by carrying a plurality of discrete magnetically active regions in a linear array. In another aspect, the invention provides a method of interrogating a magnetic tag or marker within a predetermined interrogation zone, the tag comprising a high permeability magnetic material, for example to read data stored magnetically in the tag or to use the response of the tag to detect its presence and / or to determine its position within the interrogation zone, characterized in that the interrogation process includes the step of subjecting the tag sequentially to: (1) a magnetic field sufficient in field strength to sacurate the high permeability magnetic material, and (2) a magnetic null as herein defined. Applications of such techniques are describer, inter alia, in relation to (a) identifying articles to which tags are attached; (b) accurate determination of position, as in the location of surgical probes; and (c) totalisation of purchases, where cash item carries a tag coded with data representing its nature and its price.