The ability to analyze biological specimens in three dimensions represents one of the major achievements of modern structural biology. For all but the simplest repeating structures, three-dimensional analysis is a crucial prerequisite for understanding complex biological assemblies. Macromolecular X-ray crystallography has provided three-dimensional structures at atomic resolution for proteins, nucleic acids, and viruses. Indeed, most of our fundamental knowledge on the relation of structure to function of individual biological molecules has come from these studies. Electron microscopy has been a major tool for the study of cell components. With the development of image reconstruction methods by Klug and coworkers it is now routine to study two-dimensional crystalline high symmetry objects in three dimensions at moderate resolutions (7- 20 A).