Behavior is one of two general responses available to endothermic (warm-blooded) species for the regulation of body temperature, the other being innate (reflexive) mechanisms of heat production and heat loss. Human beings rely primarily on the first to provide a hospitable thermal microclimate for themselves, in which the transfer of heat between the body and the environment is accomplished with minimal involvement of innate mechanisms of heat production and loss. Thermoregulatory behavior anticipates hyperthermia, and the organism adjusts its behavior to avoid becoming hyperthermic: it removes layers of clothing, it goes for a cool swim, etc. The organism can also respond to changes in the temperature of the body core, as is the case during exercise; but such responses result from the direct stimulation of thermo-receptors distributed widely within the central nervous system, and the ability of these mechanisms to help the organism adjust to gross changes in its environment is limited. Until recently it was assumed that organisms respond to microwave radiation in the same way that they respond to temperature changes caused by other forms of radiation. After all, the argument runs, microwaves are radiation and heat body tissues. This theory ignores the fact that the stimulus to a behavioral response is normally a temperature change that occurs at the surface of the organism. The thermo-receptors that prompt behavioral changes are located within the first millimeter of the skin's surface, but the energy of a microwave field may be selectively deposited in deep tissues, effectively by-passing these thermo-receptors, particularly if the field is at near-resonant frequencies. The resulting temperature profile may well be a kind of reverse thermal gradient in which the deep tissues are warmed more than those of the surface. Since the heat is not conducted outward to the surface to stimulate the appropriate receptors, the organism does not “appreciate” this stimulation in the same way that it “appreciates” heating and cooling of the skin. In theory, the internal organs of a human being or an animal could be quite literally cooked well-done before the animal even realizes that the balance of its thermo-microclimate has been disturbed. Until a few years ago, microwave irradiations at equivalent plane-wave power densities of about 100 mW/cm2 were considered unequivocally to produce “thermal” effects; irradiations within the range of 10 to 100 mW/cm2 might or might not produce “thermal” effects; while effects observed at power densities below 10 mW/ cm2 were assumed to be “nonthermal” in nature. Experiments have shown this to be an oversimplification, and a recent report suggests that fields as weak as 1 mW/cm2 can be thermogenic. When the heat generated in the tissues by an imposed radio frequency (plus the heat generated by metabolism) exceeds the heat-loss capabilities of the organism, the thermo-regulatory system has been compromised. Yet surprisingly, not long ago, an increase in the internal body temperature was regarded merely as “evidence” of a thermal effect.