Magnetic materials of biological origin have been an inspiration to material scientists. The singledomain magnetic materials synthesised by magnetotactic bacteria1, the magnetite (Fe3O4) in chiton teeth (and its apparent formation from an antiferromagnetic iron oxide phase)2,3, and the uncompensated surface spins on the ferrihydrite mineral core of ferritin4, are examples of biologically produced magnetic materials with unique and useful properties. Biomineralisation illustrates control over material composition, polymorph selection, orientation and morphology under mild reaction conditions5,6. A major goal in biomimetic nanomaterials chemistry is to utilise our understanding of the fundamental biomineralisation processes to synthesise target materials incorporating specišc magnetic properties by design. Control over magnetic materials synthesis using protein-mineral interfaces to control properties has generated a truly biomimetic approach to the synthesis of materials well beyond the bounds of biology. In particular, the use of protein cage nanoparticles as templates for the directed growth of magnetic nanoparticles (MNPs) will be the particular focus of this chapter.