A time temperature indicator device for measuring elapsed time after reaching a threshold temperature, formed of a planar layered structure and comprising: a rupturable reservoir containing an indicator material that is solid below the threshold temperature and liquefies above the threshold temperature, a base layer containing the reservoir, an intermediate layer comprising a migration medium, an opaque upper layer disposed on a first surface of the intermediate layer, a plastic laminate laminated to an upper surface of the base layer and to a second surface of the intermediate layer, and the plastic laminate including a passage for enabling a flow of liquid indicator material from the reservoir to reach the migration medium so that when the temperature of the indicator material exceeds the threshold temperature after activation of the device, liquid indicator material will migrate through the migration medium, producing a visible change therein; the migration medium and the passage being formed in separate layers of said layered structure; wherein the upper layer has a first window and a second window for providing a visual indication of progress of the liquid indicator material through the migration medium; wherein the device further comprises: a release layer intermediate the plastic laminate and the reservoir; the release layer operable to serve as a rupturable seal for preventing liquid indicator material in the reservoir from reaching the passage prior to activation, the rupturable seal capable of being ruptured in response to finger pressurisation of the liquid indicator material within the reservoir to dislodge the release layer, initiating flow of the liquid indicator material and thus activating the device; the planar layered structure capable of being retained folded so as to form a barrier to liquid indicator material migration, and which can be unfolded so as to permit liquid indicator material migration through the passage prior to use; and the device further comprising a removable retaining element for retaining the device folded prior to use. a rupturable reservoir containing an indicator material that is solid below the threshold temperature and liquefies above the threshold temperature, a base layer containing the reservoir, an intermediate layer comprising a migration medium, an opaque upper layer disposed on a first surface of the intermediate layer, a plastic laminate laminated to an upper surface of the base layer and to a second surface of the intermediate layer, and the plastic laminate including a passage for enabling a flow of liquid indicator material from the reservoir to reach the migration medium so that when the temperature of the indicator material exceeds the threshold temperature after activation of the device, liquid indicator material will migrate through the migration medium, producing a visible change therein; the migration medium and the passage being formed in separate layers of said layered structure; wherein the upper layer has a first window and a second window for providing a visual indication of progress of the liquid indicator material through the migration medium; wherein the device further comprises: a release layer intermediate the plastic laminate and the reservoir; the release layer operable to serve as a rupturable seal for preventing liquid indicator material in the reservoir from reaching the passage prior to activation, the rupturable seal capable of being ruptured in response to finger pressurisation of the liquid indicator material within the reservoir to dislodge the release layer, initiating flow of the liquid indicator material and thus activating the device; the planar layered structure capable of being retained folded so as to form a barrier to liquid indicator material migration, and which can be unfolded so as to permit liquid indicator material migration through the passage prior to use; and the device further comprising a removable retaining element for retaining the device folded prior to use.