This chapter discusses the effect on water budgets of six hydroregulatory mechanisms that emerge from the biophysical equations: the adjustment of oxygen extraction efficiency, the adjustment of skin resistance, microhabitat selection, activity and retreat patterns, diel activity patterns, and ocular behavior. The dynamics of heat and water flux through the biosphere are fundamentally interconnected and together determine environmental and biological processes across vast scales. The direct and indirect effects of global climate change on the temperature and water regimes of some lizards are potentially dire, and there is concern that they will lead to widespread extinctions. Biophysical approaches use physical principles to derive heat and mass fluxes through an organism as a result of the processes of radiation, conduction, convection, and evaporation. The water budgets of ectotherms are controlled by many environmental factors as well as species-specific physiological traits. The physical coupling between evaporation rates and body temperature can often mean that thermoregulatory and hydroregulatory goals are at odds.