Proteins and peptides are able to self-assemble in vivo and in vitro. In vitro, this ability can be exploited to make bionanomaterials with many potential uses. Peptides are capable of forming a wide range of structures including fibres, tubules and scaffolds. In vivo, proteins assemble to form cellular fibrous proteins, as well as being involved in protein misfolding in disease. Recent advances using X-ray diffraction have highlighted the internal structure of self-assembled proteins and peptides, showing packing of side chain residues and have enabled a deeper understanding of mechanisms of assembly.