The loss of mass in eggs during incubation was examined and evidence is presented to show that this is essentially due to loss of water. The mean fraction of water lost by diffusion throughout incubation is 0.150 ± 0.025 S D per gram of egg and 0.162 ± 0.026 S D per gram of egg content for 81 species. The water fraction of fresh eggs and of hatching eggs was examined in 32 species divided according to maturity at hatching, and found to be very similar within each category (83% in altricial 83% in semi-altricial 78% in semi-precocial 72% in precocial eggs). The 11% difference between the altricial and precocial categories is statistically significant. During incubation, dry matter is metabolized increasing the water fraction which is further increased by metabolic water production. Hence, water loss during incubation is mandatory if the relative water content of an egg at the end of incubation is to remain essentially the same as at the beginning. Equations are developed which allow one to estimate the difference between diffusive water loss and the total water loss in altricial and piecocial eggs caused by additional water loss during pipping and hatching.