Within the last ten years workers in the physical, biological, and medical sciences have become increasingly aware of the intimate relationship which exists between the chemistry of living organisms and the structure of protein molecules. The general properties of a molecule of an individual protein depend primarily upon the nature and number of the amino acid residues of which it is composed and upon the sequence in which they are arranged along the polypeptide chains. If this information alone were available for any protein it would go far toward explaining its chemical behavior and many of its biological properties; it would serve as a basis for the understanding and control of its chemical reactions, for carrying out proposed alterations of its composition, and, hence, for its possible adaptation to new and useful purposes. But this chemical information alone would not be sufficient to provide adequate understanding of those remarkably specific properties which are especially significant in determining the behavior of proteins in biological systems. These properties are directly dependent upon the spatial relationships between the carbon, nitrogen, oxygen, hydrogen, and other atoms of which the proteins are composed, and upon the nature and the direction of the forces which act between the atoms. These spatial relationships and interatomic forces are largely responsible for the ability of certain proteins to crystallize, for the architecture of protein fibers, for the specificity of enzymes, and for immunological reactions.