Protein crystallography experienced in the last twenty years a rapid development in methods and instrumentation, allowing the determination of very large and complex protein structures, particularly when combined with electron microscopy. These structures document an unlimited versatility and adaptability of the proteins' architecture, but reveal also unexpected relationships. The structures are a basis for understanding their binding specificities and catalytic properties (chemistry), their spectral and electron transfer properties (physics), and their roles in physiological systems (biology and medicine).