A sweat measuring device (1) comprising a sweat impermeable panel (40) and an adhesive skirt on one side thereof extending about its periphery to enable the panel to be secured to a user's skin is provided. The device covers a known area of a user's skin and traps sweat (42) produced by the skin under the panel. The panel (40) defines a reservoir in which sweat can accumulate and calibrations which may be associated with the reservoir to express the total sweat produced by the user as a function of the volume of sweat in the reservoir. The panel may be relatively stiff; formed by a plastics member and the skirt may extend integrally from the panel or be provided by an adhesive patch which covers the panel.