SummaryI. An arbitrary classification of avian eggshells is proposed.2. The role of the eggshell in conserving the water in eggs at oviposition is discussed. There is as yet no correlation between this property and the pore systems in avian eggshells.3. The pore systems may act as diffusion pathways and the hypothesis has been advanced that in many eggs the shells are adapted so that restriction of gaseous diffusion by mud, preening oils and nest debris is prevented.4. The mechanical properties of the shell are considered in the novel context of defence against (a) attrition that could lead to the pores being blocked with dust, and (b) cracking that would destroy the diffusion pathways noted in 3.5. The overall objective of the review was to discuss the concept that avian eggshell are adapted to fit an egg to the nest environment.