Heat- or pressure-sensitive recording medium

A heat- or pressure-sensitive recording medium comprises a microporous plastics film (1) and a contrasting underlayer (3). Heat or impact on the microporous film causes it to change from an opaque to a clear state revealing the contrasting underlayer. The medium may be used to reveal impact e.g. of a golf ball on the head of a golf club to which it is attached, or it may be used to detect temperature rises above a characteristic limit at which the film (1) becomes transparent. The underlayer (3) is preferably a coloured adhesive which is adhered to a support (5) which is coated with a pressure-sensitive adhesive (7) and a release layer (9) which is peeled off before attachment to the head of a golf club. Film (1) may be covered with a protective layer.