Hierarchical structures are assemblages of molecular units or their aggregates that are embedded or intertwined with other phases, which in turn are similarly organized at increasing size levels. The multilevel architectures are capable of conferring unique properties to the structure. Hierarchical structures are found in practically all complex systems, particularly naturally occurring ones. Synthetic hierarchical structures can be prepared from metals, ceramics, or polymers, or from hybrids of various classes of these materials. The Committee on Synthetic Hierarchical Structures made an assessment of the opportunities to prepare hierarchical structures that possess useful and unusual physical properties for civilian and military applications. It conducted case-studies by selecting natural material systems to be used as models for synthetic efforts. It reviewed the state of the art of synthetic techniques and processes for assembling synthetic hierarchical structures. The committee also characterized properties, unusual characteristics, and potential end-use applications for these synthetic systems. Finally, the committee recommended research directions to expedite the understanding of the complex phenomena involved, lead to increased coordination among disciplines, and provide direction for future activities in the field.