Protein structure influences all aspects biological function. Although there is considerable variation in the structural motifs observed in biological macromolecules they are all unified by being built from the same 20 amino acids. The differences are due to the essentially infinite number of protein sequences that may be generated from these building blocks. At a fundamental level, rigorous knowledge of the conformational properties of polypep-tide chains should lead to a complete understanding of protein structure and function. As described earlier, great progress has been made toward this goal, but much remains to be done.