Abstract: An electronic seal comprises a housing (12), a cord (14) with one end fixed to the housing (12) and a free end which can be passed through a door catch or the like and releasably connected to the housing (12), an electronic circuit within the housing (12) arranged to generate one of a number of unique codes when a switch (70) in the housing (12) is actuated on connection or disconnection of the cord (14) to or from the housing (12) and a display device (18) which can display the unique code. The cord (14) has a series of regularly spaced enlargements (36), and the interior of the housing (12) has a plurality of teeth (40) arranged so that when a length of cord (14) at its free end is inserted into the housing (12) the enlargements (36) can fit between the teeth (40) to hold the cord (14) against withdrawal from the housing (12).