A wetness monitor suitable for use in a diaper comprises an optical fibre having a core (11) and a cladding (12), borne on a substrate which is typically the outer layer (17) of the diaper. The fibre includes one or more discontinuities (13) in at least the cladding, and which may extend through the fibre； in that case the separate sections of the fibre are maintained in optical alignment. Ambient liquid enters the discontinuity preferentially, whereby the intensity of light passing through the core from a light source (14) to a light detector (15) is modified in the presence of liquid.
