Nacre, also known as mother-of-pearl, is a hard biological composite found in the inside layer of many shells such as oyster or abalone. It is composed of microscopic ceramic tablets arranged in layers and tightly stacked to form a three-dimensional brick wall structure, where the mortar is a thin layer of biopolymers (20-30 nm). Nacre is 3000 times tougher than the fragile ceramic it is made of. How its microstructure controls this remarkable performance has been the focus of intense research, because the answers may inspire novel composite designs through biomimetics. In this work the mechanics of deformation and fracture of nacre were investigated. In tension, relatively large inelastic deformations were observed, generated by sliding of the tablets on one another. The waviness of the tablets generates progressive locking, local hardening and redistribution of deformation over large volumes. Fracture experiments revealed a rising crack resistance curve, indicating robustness and damage tolerance. The toughness of nacre was correlated with mechanisms operating at the micro and nanoscales, mainly tablet sliding and energy dissipation at the tablets interfaces. These features and mechanisms, critical to the robustness of the shell, were finely tuned over millions of years of evolution and may be duplicated in man-made composite for superior performances.