This monograph describes the determination by X-ray methods and the structural analysis of one of the largest single polypeptide chain proteins, glycogen phosphorylase. It is shown that the large protein conforms to the rules deduced from smaller protein molecules and provides examples of almost every structural feature. The structure is analysed with special reference to the hydrogen bonding pattern of secondary structural elements, the distributions and interactions of the buried amino acids, the role of the water structure and the details of the subunit interface of the dimer. The results show some unusual secondary structural elements and environments for some side chain atoms. As methods for prediction of protein structures from amino acid sequences advance from assignments of secondary structural elements to predicted tertiary assemblies, knowledge of the preferred and exceptional detailed interactions of the amino acid side chains will need to be incorporated. Glycogen phosphorylase provides a wealth of examples of these interactions.This monograph is a useful reference on protein structure and function.