Success in determining the three-dimensional structure of a macromolecule by X-ray diffraction methods depends critically on the ability to obtain well ordered crystals of the macromolecule in question. Predisposition to crystallization correlates with the homogeneity of the molecules in solution. Dynamic light scattering (DLS) is particularly well suited for evaluating protein homogeneity under multiple conditions and at concentrations commensurate with crystallization conditions. This unit presents a typical protocol for DLS measurements of a protein sample, and describes approaches to improve protein homogeneity in solution.