In contrast to man-made materials, nature produces structures with remarkable mechanical properties. Seashells are compelling examples: while mostly comprised by mineralized materials, they exhibit high levels of strength and toughness. A surprisingly stiff, hard and tough hierarchical microstructure, which contains a small volume fraction of interfacial biopolymers, is responsible for creating composite materials that enable protection from predators. The high strength and toughness of seashells contrasts man-made engineering materials that sacrifice strength to achieve greater toughness. First, I will present hierarchical features and mechanisms observed from in situ microscopy experiments which enable remarkable mechanical properties. This understanding is applied to the computational design of artificial bio-inspired nanocomposites that are strong and tough. Graphene oxide-polymer composites are used to illustrate the validation of models used to design materials of interest to the material community. A strategy combining multiscale experiments and simulations, to select optimal constituents and geometric parameters, will be presented.