The secular changes in environmental temperatures and water availability driven by climate change affect the physiological performances of ectothermic animals and push some of their populations on a fast lane to extinction. The sensitivity, resilience and adaptive potential to climate change of ectotherms are all largely determined by physiological and behavioural capacities and tolerances. Individual responses to changes in temperature and water availability involve thermoregulation (i.e., physiological and behavioural regulation of body temperature) and hydroregulation (i.e., physiological and behavioural regulation of the water balance). In ectotherms, responses to water constraints remain poorly investigated when considering climate change. Yet, it is anticipated that hydroregulation and thermoregulation will influence each other through complex, possibly conflicting pathways leading to ecological responses to climate change difficult to predict by focusing solely on thermal biology. How this interplay between thermoregulation and hydroregulation influences vulnerability to climate change remains largely unknown because we lack studies that examine jointly hydro- and thermoregulation strategies involved in response to climate change. One promising and comprehensive approach to tackle this problem is to use heat, mass and water budget models that are robust and sufficiently general to be applied to a large range of study systems. Here, we will use ecophysiology and behavioural ecology to enhance our understanding of this critical facet in terrestrial ectotherms. Focusing on squamate reptiles (lizards and snakes), we will combine mechanistic biophysical models, empirical studies of physiological and behavioural traits at the individual and population levels using two model species from two French Mountain ranges, climate niche simulations for these species, and comparative analyses across all squamate reptiles. We will be able to describe and understand for the first time the covariation patterns between thermoregulation and hydroregulation, and to investigate and improve our capacity to predict ecological effects of two global change pressures (temperature and water) in terrestrial ectotherms. This approach will provide new insights on the role of proximate functional traits in determining species distribution and sensitivity to climate change, and translate into knowledge applicable in other terrestrial ectotherms and wildlife management.