During embryonic development of homeothermic animals like birds and mammals transition from ectothermy to endothermy occurs especially in precocial species of both taxa. Based on some evolutionary aspects of the development of endothermy the review focuses on the prenatal development of endothermy and of the thermoregulatory system using the precocial bird as a model. During final incubation precocial bird embryos have all the prerequisites to respond to environmental (temperature) influences in a nearly appropriate way. Autonomic, neuronal and molecular mechanisms underlying the development of endothermy are established and the transition of the thermoregulatory system from a control system without feedback into a system with feedback mechanisms occurs. Precocial bird embryos are endothermic, but not homeothermic if incubation temperature decreases below the normal level, which seems to be associated with summit metabolism. At increased incubation temperatures the embryos are able to stabilize their body temperature. Therefore, above normal temperatures, homoeothermy can occur over a limited temperature range.