Abstract: A device for indicating the time integral of a monitored parameter comprising a strip of electrochromic material (2), e.g. tungsten trioxide, an electrode (3) at one end of the strip and an electrolyte (4) arranged so that, upon activation, electrons pass from the electrode (3) to the electrochromic material (2) causing the latter to change color so that a color change boundary (7) advances along the strip, the rate of advancement being determined by the supply of electrons between the electrode (3) and the color change boundary (7), which in turn is responsive to changes in the monitored parameter. The device may be used in the form of a self-powered cell needing no power source, as a time temperature indicator suitable for application to perishable commodities.