Macromolecular crystallography is the structure determination by X-ray diffraction methods using crystals of macromolecules such as proteins, glycoproteins, carbohydrates, nucleic acids or complexes of these macromolecules. Macromolecular crystallography is most often referred to as protein crystallography (PX) because most research is carried out on this family of biological macromolecules. For convenience, if not for correctness, the term 'protein' is used throughout to represent the macromolecule of interest in these experiments. Other experimental methods such as NMR and electron microscopy can also be used to determine the molecular structures of biological molecules, but PX represents the most successful method with ~85% of ~29,000 structures determined by this method. PX is especially powerful for determining macromolecular structures with more than one chain or more than 100 residues (~95% of ~15,000 and ~23,000 structures, respectively, RCSB Protein Data Bank, Dec 2004 (1)).