This chapter focuses on the ethological aspects and proximate mechanisms of thermoregulatory behavior of lizards, occasionally drawing from research on other reptiles. It discusses future research areas could incorporate a phylogenetic approach to analyze the adaptive significance of specific thermoregulatory behaviors in lizards. Changes in external environmental factors may alter peripheral or internal states, with the changes subsequently being perceived by sensory receptors. An organism must maintain its body within a restricted temperature range for optimal function, and it is through the process of thermoregulation that the required range of temperatures is achieved. Fitting within the context of neurophysiological control, there are two principal control mechanisms for behavioral thermoregulation: proportional control, used for fine-tuning body temperature, and on–off control, used for overt thermoregulation. Temperature is a driving force in many physiological processes such as reproduction, gestation, digestion, survival, and growth.