Engineered proteins as multifunctional materials. Living organisms have engineered remarkable protein-based materials through billions of years of evolution. As multifunctional materials, these show unparalleled mechanical, optical, and electronic properties and have served as inspiration for scientists to study and mimic. This issue of MRS Bulletin covers the mechanical and rheological properties of engineered structural protein materials and nanocomposites, advancements in the synthesis and assembly of protein materials, and recent developments in the processing of these materials. The work in the inset on the cover demonstrates that the specifi c penetration energy of a silk/graphene oxide bionanocomposite is signifi cantly higher than that of Kevlar because of the nanoscale morphology and the interactions between the two components. The layer-by-layer bionanocomposite image depicts its brick and mortar structure and an approaching silica sphere in a projectile impact test. Image courtesy of Washington University in St. Louis. The background shows an atomic force microscope image of co-assembled silk and MXene fl akes (Z-scale: 20 nm). The false color shows the silk corona morphology. Image courtesy of Michelle Krecker. See the technical theme that begins on p. 999.