<p>This article addresses recent advances in using <i>de novo</i> protein design to create coherent interfaces between proteins and inorganic materials, either through protein self-assembly on crystal lattices or through directed nucleation and growth of crystals by protein scaffolds. Inspired by natural protein–crystal interfaces, we focus on designed helical repeat proteins that present a repeating pattern of charged amino acid residues&#xa0;that&#xa0;epitaxially&#xa0;match a target&#xa0;inorganic crystal lattice. We describe the use of <i>in situ</i> imaging and spectroscopic methods to investigate both the assembly of these proteins and their ability to direct crystal nucleation and growth. The findings reveal the importance of surface charge, facet-specific binding, solvent organization, and, more generally, the balance of protein–substrate–solvent interactions in determining how organized protein–materials interfaces emerge. The&#xa0;results &#xa0;highlight&#xa0;the vast potential of protein design in materials science and inform our understanding of&#xa0;the mechanisms by which interactions between biomolecules and inorganic surfaces lead to unique materials and morphologies.</p> Graphical abstract <p></p>