The benefits of using polymer and polymer composite gears are numerous and it is these in part that is fuelling the resurgent commercial interest in them for metal replacement applications, thought to be in the region of £160M per annum. This interest is further driven by the increasingly important need for considerate energy consumption and the drive for greater product efficiency. As the science and technology associated with polymeric gearing develops, so too do the complexities and demands of the application. Although traditionally employed in low load and speed applications and motion control, such advancements are pushing the limits of high performance polymer applications to unprecedented levels where hitherto high temperature and corrosive environments would have excluded them. Thus, the complex kinematics of gearing and the extreme environments in which polymer gears operate make such applications ideal candidates for material manufacturers to demonstrate the superiority of their proprietary polymers. Despite this resurgent interest, polymer gears have received only a fraction of the attention received by their metal counterparts. As a result, those choosing to specify polymers in demanding applications are faced with a lack of reliable design and testing data, with current standards and guidelines based on metallic dogma. Given the fundamental differences between the two materials, this has been proved to be inadequate. This thesis aimed to address a variety of performance aspects of polymeric gears, particularly focusing on temperature and tribological related effects. At its foundation was the previous research conducted by the Power Transmission Laboratory, at the University of Birmingham. Five research areas were covered, they were: - The analysis and estimation of bulk temperatures in polymer spur gears - Contact analysis of polymeric gears - Acoustic emissions and transmission error analysis: - External lubrication analysis with respect to measured efficiencies and surface damage: -A design and rating algorithm for polymeric gears: The research utilised empirical, numerical and analytical techniques to improve the understanding of the behaviour of polymeric gears, such that their performance and reliability could be improved. In addition to this, recommendations were given for future research directions.