Not so long ago Roentgen-ray crystallography was a rather esoteric science reserved for a fairly small group of specialists. Perusal of the recent literature shows that the number of publications per year con- cerned with crystal structure analysis by x-ray diffraction has increased by a factor of ten in the last ten years. This great increase can be at- tributed to a number of factors. An analysis of a molecule which may contain eighty atoms, excluding hydrogen atoms, involves the collection of about 6000 independent reflections. Before the advent of the automatic diffractometer, an instrument which records intensity data while the investigator is free to be occupied otherwise, the estimation of the intensities of the reflections by eye or by photometer was a formidable task. Even if such a large number of data were collected, processing these data was an impossible task until the development of high-speed computers. A third factor was the advancement made in the theory of structure analysis such as the use of isomorphous replacement, anomalous dispersion and direct methods for phase determination. As a result, at the present time it is possible to determine the structure of virtually every material which forms good single crystals, regardless of the number of atoms or whether or not heavy atoms are present.