The world we interact with is dominated by intertwined materials known as composites. Unlike in metal alloys or polymer blends, where the atoms or molecules are intimately mixed, components of a composite material retain their individual identities, and their careful selection and combination maximizes certain sets of properties. Here, too, another facet of composite materials emerges--they typically have stronger and weaker directions (i.e., asymmetric properties), as they are designed to fulfill the needs of specific applications. The benefits of combining materials are easily learned from nature. Natural composites such as nacre, wood, and teeth have impressive properties arising from their hierarchical structures, especially as they are generated from easily obtained starting materials that often have limited capabilities.