Abstract: A sensor device, for fitting to, for example, a drum brake shoe includes a loop of wire which can electrically contact the drum to provide a signal to indicate wear. After a predetermined amount of wear of the shoe lining, the wire contacts the drum on each brake application to provide an intermittant warning signal. When the lining becomes dangerously worn, the loop is severed and the ends spring apart so as to maintain contact with the drum and provide a continuous signal. The wire is mounted in a housing of plastics or of sheet metal, which may be a snap-fit in to the shoe structure.