This volume is the twelfth classified bibliography of organic, organometallic and metal complex crystal structures prepared by the Cambridge Crystallographic Data Centre and published jointly with the International Union of Crystallo graphy. The previous eleven volumes covered the years 1935-79; the present volume provides references principally to structure analyses reported in the literature during 1979 and 1980. A few structures reported prior to 1979 and omitted from earlier volumes are also inc1uded here. Vo1ume 12 contains 3929 references to 3836 distinct chemica1 compounds with 1939 cross-reference entries. During 1979-80 some 90% of references were obtained via direct in-house scanning of 51 journals; the remaining material was located by scanning Chemical Abstracts and Bulletin Signa/etique. The tab1e be10w summarizes the 1980 cut-off dates for the 25 direct-scan journals yielding the most entries in Volume 12. Other journals are ca. 95% complete for 1979, ca. 65% complete for 1980. The following conference proceedings are included in this volume: 5th and 6th European Crystilllographic Meetings, Copenhagen 1979 and Barcelona 1980; American Crystallographic Association Winter and Summer Meetings, 1980. The indexes presented in V olume 12 continue the system established in Journal Issue Page Year Entries Acta Crystallogr., Sect. B. 9 2191 1980 655 J. Amer. Chern. Soc. 15 5101 1980 328 Inorg. Chern.