The first stage in the determination of a crystal structure is the measurement of the intensities of the Bragg reflexions. This book is concerned with counter-methods of measuring these intensities. The use of counter methods has spread rapidly in recent years with the development of quantum detectors and of automatically controlled diffractometers. The authors of this book were among the first workers to design and operate automatic X-ray and neutron diffractometers; they have visited laboratories throughout the world where such instruments have been installed. Originally published in 1966, this book covers all aspects of single crystal diffractometry. A discussion of the geometrical principles of diffraction by single crystals is followed by a description of diffractometers and of their component parts, including radiation detectors and detector circuits.