It is well known that ambient temperature has a major impact on poultry production. Heat and cold stress can occur at temperatures above and beneath the upper and lower critical temperatures, respectively. In the case of heat exposure, feed consumption decreases, resulting in highly detrimental effects on growth of broilers and egg production and shell quality in laying. Exposure to low temperatures results in increased feed consumption, but also results in decreased performance in broilers and laying hens. Thus, both may lead to economic losses and alter animal welfare. Different strategies have been studied in order to reduce the detrimental effects of heat and cold stress. Temperature acclimatization during embryogenesis has received experimental attention as an effective way. This phenomenon refers to chickens subjected to higher or lower incubation temperatures that may acquire tolerance to higher or lower temperatures during postnatal life, respectively. This hypothesis is based on the assumptions that: 1. Temperature experienced during embryonic development may influence physiological responses to the environment and 2. It may have positive effects by improving thermotolerance. Previous studies showed that temperatures during critical developmental phases that affect gene expression have persistent effects on thermal acclimatization of chickens. This critical period is linked to the development and maturation of the hypothalamus-pituitary-thyroid axis and hypothalamus-pituitaryadrenal axis. The mechanisms involved in thermotolerance acquisition include changes in pathways regulating energy metabolism resulting in a change in heat production, modifications in stress markers, changes in anti-oxidant pathways and membrane alterations. The aim of this paper is to review 1. Whether and how temperatures experienced during embryogenesis affects temperature tolerance of broilers and laying hens and 2. The physiological mechanisms underlying adaptive temperature response.