Fowls are endothermic species controlling body temperature (Tb) within a relatively narrow range, despite moderate to extreme changes in environmental conditions. Tb is recognized as the main characteristic that reflects thermal status. Significant developments in the genetic selection of domestic fowl enhanced performance-improved feed efficiency and increased metabolic rate, leading to low capability to balance energy expenditure and maintain body water balance under extreme environmental conditions. This chapter will focus on the effect of combinations of environmental conditions (temperature, ventilation, relative humidity) on thermal status and performance parameters of broilers, turkeys and laying hens. Furthermore, it will offer the use of epigenetic temperature adaptation during embryogenesis as a tool to improve thermotolerance and performance.