In Nature, there are many examples where inorganic species and materials are combined with proteins, and these vary in function, e.g. catalysis, often in oxidation reactions or storage to regulate uptake and release of ionic species, but also for the formation of composite biological materials such as bone and teeth during biomineralization processes.^1–3  Therefore, from a material science point of view, there are many interesting possibilities to be explored using proteins combined with inorganic materials to develop new applications, synthetic methods, material properties and morphologies.^4–89 

In this chapter, bionanocomposites are addressed where a protein is used either to act as a nanoreactor for particle synthesis, as a template for new nanomaterial development or for biofunctionalization rendering nanomaterials more biocompatible. For many of these approaches, multicomponent protein structures such as cage proteins and virus particles are used, which will be the focus here. These multicomponent structures have various capabilities for hosting inorganic materials both on the surface [Figure 2.1(A)] and inside the assembly [Figure 2.1(B) and (C)]. These are general approaches that can be used in combination with assembled protein structures including cage proteins and virus structures.