After some general remarks on protein structure, there follows a discussion on primary, secondary, and tertiary organization. The account of primary structure includes a discussion of the conformation of disulfide bonds. Types of helices, sheets, and turns are described in the section on secondary structure, followed by a discussion of super-secondary structure and the effects of metals and prosthetic groups of protein fold. The crux of the review lies in an examination of tertiary structure, or specifically of domains that are defined, in part, as functional units within a polypeptide chain. An assembly of domains can in turn result in a protein whose function is quite sophisticated. Some consideration of domain recognition is given in the section on taxonomy and in the appendix. The key part of the tertiary structure section concentrates on a taxonomic protein classification dependent not only on structure but also on function. A discussion of the requirements by quaternary structure on a fold are omitted in this review. Finally, no review of this kind can escape a discussion of evolutionary convergence and divergence.