The scope of this book is quite simply denned; we have tried to cover the subject of crystal-structure determination from the stage at which a set of structure amplitudes has been obtained to the final accurate positioning of the atoms. Some of the steps-such as the calculation of structure factors and the summation of Fourier series-are routine ones, and are described first, but the last five chapters are concerned with the more difficult problems of the derivation of atomic positions, and in these chapters we have attempted to preserve some degree of logical order in the treatment. Nevertheless, we have emphasized throughout the book that each problem requires individual treatment, and that no single sequence of methods is likely to prove universally successful.