This volume is the fourth c1assified bibliography of organic and organometallic crystal structures prepared by the Crystallographic Data Centre, University Chemical Laboratory, Cambridge, and published jointly with the International Union of Crystallography. The first three volumes covered the years 1935-1971. The present volume provides references principally to compounds whose struc­ tures were reported in the literature during 1971-1972. A few structures published prior to 1971 and omitted from the previous volumes are also inc1uded. The arrangement of entries in the 86 chemical c1asses is identical with the previous volumes and the reader is referred to the Introduction in Vol. 1 or Vol. 2 for a description of the practical use of the bibliography. There are three cumulative indexes in the present volume: formula, transition metal and author indexes. All three cover the period 1935- 1972 and give references to entries in Vols. 1-4. The bibliography and indexes were prepared, checked and printed by computer techniques described in the previous volumes. Magnetic tapes of the four volumes are available and anyone interested should contact the Centre for further details. In the present volume we have attempted to improve the cut-off date by special arrangement with the Centre National de la Recherche Scientifique, Paris, France. Und er this arrangement reprints ofpapers containing crystallographic data are now sent directly to the Crystal­ lographic Data Centre, Cambridge, at the same time as they are sent out to abstractors preparing material for the Bulletin Signaletique.