Abstract: A selectively activated time-temperature integrating device for providing a visually observable indication of cumulative thermal exposure, in which a viscoelastic material migrates into a diffusely light-reflective porous matrix at a rate which varies with temperature to progressively change the light transmissivity of the porous matrix and thereby provide the visually observable indication. The viscoelastic material can comprise a pressure sensitive adhesive. The visually observable indication can comprise latent indicia which initially are not visible and become visible, obscurable indicia which initially are visible and become obscured, or a combination of both. Various actuating means for selectively switching the indicator from an unactivated state to an activated state are described.