Embryo's from different breeds, breed age, egg size, storage time, etc. may require different incubation techniques. Aim of this study is to gain insight into embryo development during incubation in order to be able to adapt incubation conditions to the requirements that embryo's have for optimal development, under all circumstances. [Results:] - As well low incubation temperature at the start of incubation and at the end of incubation reduce heat production; this has direct consequences for embryo temperature, hatchability, embryo development and post hatch performance. - Large eggs produce more metabolic heat than small eggs; environmental temperature needs to adapted in order to incubate eggs from different weight classes at the same internal egg temperature.