||During incubation, an eggshell temperature (EST, as a reflection of embryo temperature) of 37.8°C was long considered to be optimal for broiler embryonic development. However, an EST of 36.7°C (Cold) from embryonic day (E)15 onward may result in a more developed heart at hatching than 37.8°C EST throughout (Control; Maatjens et al., 2016). Maatjens et al.’s study was performed in large incubation chambers with low air velocity, unlike commercial practice. To study Cold EST in a commercial situation with high air velocity and egg density, three trials were conducted. In all trials, EST for Cold was maintained at 37.8°C, decreased to 36.7°C with varying profiles in the last week, and then maintained at 36.7°C till hatching. Cold treatments were always compared to Control (37.8°C EST throughout incubation). 3,000-10,800 broiler eggs from a 30-39 week old parent flock were used. Firstly, EST was decreased within 30 minutes, on E15. Compared to Control, Cold resulted in 2.5x higher embryonic mortality around the time of the EST decrease (P = 0.015), 2.2% more second grade chicks (P = 0.049), and chicks were 0.4cm shorter (indicating lower development) at hatch (P = 0.001). Possibly, the EST decrease happened too early or abruptly. Secondly, EST was decreased gradually in 1 day, from E16-E17. No differences were found in hatchability (P = 0.68) or chick length (P = 0.93), but embryonic mortality around the time of the EST decrease tended to be 1.7x higher for Cold than for Control (P = 0.070). It was thought that an even slower EST decrease may optimize Cold further. Thirdly, EST was decreased using three different profiles. EST was decreased gradually from E16-E17, or quickly (to 36.9°C on E17) and then slowly (to 36.7°C on E18), or slowly (to 37.5°C on E17) and then quickly (to 36.7°C on E18). Embryonic mortality, hatchability, and navel quality did not differ between the Cold profiles and Control (P > 0.29). To conclude, results differed from the low air velocity trial situation. When EST was decreased from 37.8°C to 36.7°C abruptly or too early in development, hatchability and chick quality decreased. A slow transition from E16-E18 can result in chick quality and hatchability similar to Control. Knowledge on these optimal EST decrease profiles can be used to further investigate the effect of Cold incubation during the last days of incubation on post hatch performance and possibly apply it to commercial practice.