The scope of this book covers the use of synchrotron radiation in the X-ray analysis of single crystals of proteins, nucleic acids and viruses. The impact of this new X-ray source with its polychromatic nature and associated high intensity and fine collimation has brought important advances in the field of macromolecular crystallography. It has extended structure determinations to higher resolution, allowed use of smaller samples and larger, more complex, unit cells. Several new methods have come to the fore and some old methods have been revived. Firstly, the Laue method is being developed and used now for quantitative, time resolved analysis of structure. Secondly, variable wavelength methods are being developed and used for phase determination for metallo-proteins or derivatised proteins. Thirdly, the diffuse scattering is being measured more easily and procedures for analysing it are being developed in order to study molecular flexibility; hopefully its use will be increasingly widespread but at present it is the least developed of these three methods. The availability of the synchrotron is a very modern development but it has reopened fundamental questions of which crystallographic method to use. It is interesting to wonder what von Laue, W. H. and W. L. Bragg and the other early pioneers would have made of the synchrotron instead of starting with the X-ray emission tube.