The presented thesis contains a comprehensive study of the thermo-mechanical behaviour of polymer spur gears during running, based on which a predictive model was built, for the evaluation of the temperature rise in the gear structure due to generated heat losses. This rise was found to be related predominantly to frictional heat losses and can be described as a superposition of two components, i.e. the local flash and long term nominal temperature rise. In our analysis, we specifically focused on a pair of thermoplastic polymers commonly employed in polymer gear drives, i.e. POM and PA66, and on gear running in non-lubricated conditions. The developed numerical model was structured in a decoupled configuration, where, based on a mechanical finite element analysis of the gear meshing process, it was possible to determine the resulting frictional heat generation and subsequently construct a thermal analysis procedure for the evaluation of the temperature rise during the meshing cycle, as well as during longer running periods. The model was validated based on a detailed experimental procedure carried out on a custom-developed gear testing rig by using a high speed thermal imaging camera. The comparison between the experimental and analytical results revealed a satisfactory correlation between the measured temperature response of the gear pair and the model predictions.