Abstract: A tag for securing to a person or object, comprising at least one fibre optic sensor having a plastics optical fibre that changes optical transmission characteristics with a change in temperature, and a sensor unit coupled to the optical fibre. The sensor unit is configured to detect change in the characteristic of optical transmission. The optical fibre is coupled to a locking strap to extend around a portion of a person or object. A detectable increase in attenuation of light transmitted through the optical fibre may occur when the temperature of the optical fibre is increased from a normal use temperature to at least a threshold temperature, which may be in the vicinity of the softening temperature of the optical fibre. The optical fibre may be used in electronic tags for offender monitoring to ensure the continuity of the strap, and to detect softening and stretching of the optical fibre.