Closely follows an actual structural determination. After some introductory material on the nature of x--rays, the diffraction process, and the internal geometry of crystals, the selection and preparation of a crystal are considered. Techniques of measuring raw x--ray data are covered, plus their reduction into a useable form. The second part discusses both traditional and novel methods of solving the "phasea a problem, the principal difficulty in x--ray structure determination. The third part considers how to extract the most information from the data and how to evaluate its reliability. Finally, there is a discussion of sources of error in practice and interpretation.