Publisher Summary Among the methods available for the characterization of globular proteins, small-angle X-ray scattering (SAXS) is particularly powerful. This method is capable of yielding the radius of gyration and, when used on the absolute-intensity scale, the molecular weight, hydrated volume, surface-to-volume ratio, and the degree of hydration of a particle in solution. To obtain this information, it is necessary to measure two auxiliary parameters: the concentration and the partial specific-volume of the protein. In addition to the molecular parameters, the thermodynamic parameters of interacting systems can be obtained, such as the association constants of aggregating subunit systems and the degree of preferential interaction of proteins with components of mixed solvent systems. In the case of highly concentrated solutions, in which there are strong long-range intermolecular interactions, SAXS can be used to determine the radial distribution function of the interacting macromolecules in solution. This, in turn, yields the interaction potentials characteristic of the operative forces. At still higher concentrations, distinct bands may appear as the system gradually becomes ordered and X-ray scattering passes over into small-angle X-ray diffraction.