Protein crystallization is the most difficult and time-consuming step in the determination of a protein's atomic structure. As X-ray diffraction becomes a commonly available tool in structural biology, the necessity for rational methodologies and protocols to produce single, high-quality protein crystals has come to the forefront. The basics of protein crystallization conform to the classical understanding of crystallization of small molecules. Understanding the effect of solution variables such as pH, temperature, pressure, and ionicity on protein solubility allows the proper evaluation of the degree of supersaturation present in protein crystallization experiments. Physicochemical measurements such as laser light scattering, X-ray scattering, X-ray diffraction, and atomic force microscopy provide a clearer picture of protein crystal nucleation and growth. This ever deepening knowledge base is generating rational methods to produce protein crystals as well as means to improve the diffraction quality of such protein crystals. Yet, much remains unclear, and the protein crystallization research community will be quite active for many years to come.