Behavioral thermoregulation is well studied in reptiles but has not previously been known to exist in butterflies, where it seems to be widespread and perhaps universal. Like reptiles, butterflies are heliotherms, deriving their heat almost exclusively from the sun. For reception of heat they make much use of their wings. Blood circulates in the wings, making them effective heat exchangers whose efficiency is improved by modifications of structure, color and pattern. Behavioral acts (often group—specific) for heat gain include dorsal, lateral and body basking, and ground contact. Behavioral devices for heat loss include shade seeking and probably normal and excessive respiration (evaporative cooling). In optimal air temperatures a series of minor adjustment devices permits normal activity with little concession to temperature control: sum minimizing, irradiation balancing, and wing opening. Five major thermoregulatory behavior patterns can be distinguished: minimal activity, matutinal warming, vesper warming, vernier control and cooling dominant. One or another of these is in operation at all times. Butterflies and reptiles share many behavior devices but moths differ considerably from both. They are myothermus, their major heat source being muscular energy. Heliothermic butterflies and myothermic moths are compared and contrasted with respect to the ecological significance of these specializations, particularly in feeding habits. Some of the many remaining problems are discussed, most important of which is the necessity for obtaining actual body temperatures to amplify the observational data of the present paper.