Virus capsids, and other related protein cages, are remarkable examples of natural supramolecular assemblies. These systems self-assemble from a distinct number of subunits to form well-regulated and often highly symmetrical quaternary structures. They exhibit sophisticated functionalities such as protection of cargo molecules (e.g., their genome), targeting host cells and triggered release of their cargo. Scientists have drawn inspiration from protein cages and utilize them as platforms to explore this new area of materials science. The interior cavities of protein cages can be used as size-constrained reaction volumes to encapsulate cargo molecules. The exterior surfaces can be engineered to direct the interaction of the cage to the external environment such as for cell-targeting and programmed stimulation of immunological responses. Individual protein cages are also viewed as ideal building blocks for higher-order assembly of ordered arrays.