**5. Conclusions**

Thermal manipulation is an important approach that has been deeply studied due to its role in alleviating the effects of heat stress on broiler chickens. The success of this application depends on duration and strength of exposure in addition to embryo age at the timing of TM. The ideal embryonic stage for TM is between E7 and E18, in which thermoregulatory roles are enhanced. While intermittent TM has no adverse effects on embryonic development, hatchability, and hatching quality of broiler chicks, continuous TM depresses the above parameters. High-temperature (39–39.5°C) TM accelerates hatching time, shortens the incubation period, but has no significant effect on embryoinc development, hatchability and chick quality compared to low TM and incubation at standard temperature (37.8°C). Interestingly, in some studies, TM below 37.8°C was shown to increase chick weight at hatch compared with TM above 37.8°C. Furthermore, there is need for more comparative studies between low and high-temperature TM and the duration of TM because on addition to the available studies being insufficient, their results are also controversial. Additionally, a meta-analysis to provide an insight into contradicting results of TM application is thought of as a sound option. Also, there is need to continue studies on TM to identify the exact duration and intensity of TM and embryonic age to obtain higher hatchability and improved chick quality.
