A way of monitoring tags within a monitored volume is disclosed. A plurality of tags each has a store holding a unique identification comprising a first level code and a second level code. Each tag also has a comparator for selectively comparing incoming codes with one of the first level code and the second level code. A tag detector transmits a plurality of codes in sequence to uniquely identify one of the tags. The tag detector transmits a first level code and only transmits a second level code if at least one of the tags with a first level code matching the transmitted first level code has responded. The provision of first and second level codes allows the length of each code to be reduced and still allow a large tag population.
