Dynamic light scattering (DLS) has become one of the most useful diagnostic tools for crystallization. The main purpose of using DLS in crystal screening is to help the investigator understand the size distribution, stability, and aggregation state of macromolecules in solution. It can also be used to understand how experimental variables influence aggregation. With commercially available instruments, DLS is easy to perform, and most of the sample is recoverable. Most usefully, the homogeneity or monodispersity of a sample, as measured by DLS, can be predictive of crystallizability.