x Ray crystallography is currently the most favoured technique for structure determination of proteins and biological macromolecules. Increasingly, those interested in all branches of the biological sciences require structural information to shed light on previously unanswered questions. Furthermore, the availability of a protein structure can provide a more detailed focus for future research. The extension of the technique to systems such as viruses, immune complexes, and protein-nucleic acid complexes serves only to widen the appeal of crystallography. Structure based drug design, site directed mutagenesis, elucidation of enzyme mechanisms, and specificity of protein-ligand interactions are just a few of the areas in which x ray crystallography has provided clarification.