The sea shells are interesting material from the mechanical point of view as they show superior properties than their constituents. Two major shell structures- nacre and crossed lamellar are reviewed and compared in this book on the basis of mechanical properties -tensile, compressive and bending strength, fracture toughness and hardness and their adaptation to the loadings encountered during lifecycle of the molluscs. This book further illustrates the microstructure, shell growth and the modeling of nacre and crossed lamellar structure. Adaptation of various structures to different environments is also discussed. The thermal analysis elucidates the role of organic material in enhancing the mechanical properties.