Abstract: A personnel monitoring system includes a tag worn by each monitored person. The tag repeatedly emits digital radio signals, and a predetermined bit in the signal changes with time according to a predetermined pattern. Monitoring apparatus detects the radio signals and sends an absence alarm signal if the signals cease. The monitoring apparatus detects spurious, forged signals which do not replicate the predetermined pattern of variation and provides a forgery alarm signal. The tag may be attached to the person by a strap bearing parallel electrical conductors. Attempts to remove the tag by servering the strap cause electrical contact between the conductors. Such contact triggers a detector, which in turn alters the ratio signal.