A time-temperature indicator label for measuring the length of time to which a product has been exposed to a temperature above a pre-determined temperature is provided. The period of time of exposure is integrated with the temperature to which the indicator is exposed. The label is a composite of a plurality of layers adapted to be adhered at its underside to a product container. The label includes a printable surface layer, a longitudinal wicking strip that is adhered underneath the surface layer substantially at the opposite extremities only of the wicking strip and a lower substrate layer forming an envelope with said surface layer. A heat-fusible substance, which melts and flows above a pre-determined temperature, is applied on the surface of the wicking strip contiguous to at least one of the ends of the wicking member. When the heat-fusible substance is exposed to a temperature above the pre-determined temperature, the heat fusible substance flows along the length of the wicking member. The label has a printable surface layer and is sealed at its peripheral edge to the peripheral edge of the substrate layer. These layers encapsulate the wicking member and the heat-fusible substance. The surface layer is provided with a sight window at an intermediate location over the wicking member through which the progress of flow on the wicking member is observed.