Temperature is one of the most important environmental variables impacting organisms. For endotherms, temperature can be extremely testing since they need to maintain a constant core body temperature at any given ambient temperature. However, endotherms can survive both in very hot areas and very cold areas and mammals are, for example, found throughout the world and widespread species may face temperature differences of more than 80C over their distribution range. Recent changes in climate are likely to have great impact on thermoregulatory abilities and with increased regional temperatures due to anthropogenic climate change more and more species are being pushed to, or beyond their thermal limits in parts of their distribution, making an understanding of adaptation to temperature particularly important. The following series of studies examine multiple aspects of thermal adaptation in the American red squirrel (Tamiasciurus hudsonicus). The first chapter asks if the distribution of the species is limited by temperature. Indeed, three of the major predictors of the species distribution reflect temperature, i.e. mean annual temperature, minimum temperature of the coldest month and maximum temperature of the warmest month. Knowing the importance of climate in determining the species niche helps us understand the selective pressures it asserts. The second and third chapters discuss adaptations found in the morphology of T.hudsonicus that relate to temperature. Both whole body and skull dimensions vary with temperature, with smaller animals being found in colder areas, either as a response to reduction of heat loss in cold conditions or increased heat loss in hot conditions. This is consistent with Bergmann’s Rule, but with regards Allen’s Rule, only one protuberance or appendage – the nose - was found to show an adaptive trend and that shows a relationship with humidity rather than temperature. The fourth chapter examines variation in gene expression levels in response to temperature and elevation. The gene expression responses suggest that T.hudsonicus has a decreased metabolic activity in cold conditions compared to warm (or alternatively increased metabolic activity in warm conditions) and shows greater metabolic responses to increased elevation than to temperature. Overall T.hudsonicus is found to live within limits determined by temperature and shows both morphological and gene expression response to temperature.