A new passive temperature monitoring device, the Integrating Thermal Monitor (ITM) has been developed for measurements in reactor environments. The operation of the ITM is based on the thermal annealing behavior of latent particle tracks in Solid State Track Recorders (SSTRs). Several SSTR materials have been identified which have time/temperature annealing sensitivities for latent tracks which match the environments of the nuclear power plant measurement locations of interest. Of these, nine ITM materials have been calibrated at constant temperatures of 30, 40, 50, 60, 70, 80, 90, 100, 150, and 175 °C for periods ranging from hours to more than 4 years. Detailed response measurements have been obtained for more than 600 ITM samples to establish a calibration data base against which ITM response can be measured. A model has been developed for ITM response as a function of time and temperature, and this model is used to interpret the response of ITMs deployed at power reactors. Methods for using ITMs to determine Arrhenius Equivalent Temperatures for equipment components operated in unknown temperature environments are described, and ITM performance to date in nuclear power plant applications is also summarized.