The dynamical theory of diffraction has witnessed exciting developments since the advent of synchrotron radiation. This book gives an up-to-date account of the theory of diffraction and its applications. The first part serves as an introduction to the subject, presenting early developments and the basic results. It is followed by a detailed development of the diffraction and propagation properties of X-rays in perfect crystals and by an extension of the theory to the case of slightly and highly deformed crystals. The last part gives three applications of the theory: X-ray optics for synchrotron radiation, locations of atoms at surfaces, and X-ray diffraction topography. The book is richly illustrated and contains a wide range of references to the literature. It will be a useful reference work for graduate students, lecturers, and researchers.