The term protein-based polymer has its origin in a symposium containing that name organized by D.L. Kaplan, M.T. Marron, and D.A. Tirrell (1990). As the term is used here, it refers to polymers comprised of repeating peptide sequences in which the repeats may be as few as two residues or as many as hundreds of residues. In the latter case, properties of protein-based polymers and globular proteins naturally merge. For the former case, there are now known many examples of repeating peptide sequences in proteins. These occur most commonly in proteins that fill structural roles as opposed to the frequently discussed catalytic, transport, or transductional roles of globular proteins.