The shell is the only part of the egg that is not consumed, yet its quality is critical to the marketing of eggs. This is especially true as recent developments in egg production systems towards free-range hens and longer production cycles have affected shell quality, particularly its mechanical properties. Here we describe the structure and composition of eggshells and review how eggshell minerals are supplied and regulated to provide the large quantities required for shell formation. The organic matrix also plays a fundamental role, interacting with the mineral phase to give the eggshell its ultrastructure and resulting mechanical properties. Here we describe the composition of this matrix and the mechanisms involved in eggshell formation. The role and importance of amorphous calcium carbonate (ACC) is underlined and detailed. Finally, we describe how all this knowledge could be mobilized to improve eggshells, especially as hens age.