There have been many advances in x-ray crystallography since the production of the third edition of this book, and the authors have endeavoured to introduce a number of them into this edition. The overall plan of the book has been maintained because we believe that it has been well received in the academic community, but substantial revisions have been carried out and new material and chapters added. In particular, we have extended the discussion of the theory of x-ray diffraction and added new chapters on structure determination from powder data, on macromolecular crystallography, and on computational procedures in x-ray crystallography. We consider that x-ray crystallography is a universal tool for studying molecular structure, a view upheld by the pioneers in the subject, notably W.H. & W.L. Bragg, J.D. Bernal, Dorothy Hodgkin (nee Crowfoot), Kathleen Lonsdale (nee Yardley), and Linus Pauling, so that the broadening of the scope of the text in this way is fully justified. We have maintained the practice of devising problems to illustrate the work of each chapter, and have provided detailed, tutorial solutions. The appendices contain mostly mathematical procedures related to the material of the main text. This edition is accompanied by a suite of computer programs on a compact disc. The programs enable the reader to participate fully in many of the aspects of x-ray crystallography discussed in the book. In particular, the program system XRAY* is interactive, and enables the reader to follow through, at the monitor screen, the computational techniques involved in single-crystal structure determination, albeit in two dimensions. Several sets of x-ray data are provided for practice with this system. There are many good things about the book; the most important is the wide range of examples and applications of solids that are described, which gives the book an interesting slant. Another is the simple use of a single colour, plus black, in many of the diagrams, which makes many of them much easier to understand - this was approved by my own students. Every chapter has a problem and answer section, which is also very welcome, and a further reading list.