This chapter discusses the occurrence, isolation, structure elucidation, spectroscopy, synthesis, and transformations of spirobenzylisoquinoline alkaloids. These represent a rapidly growing group of isoquinoline alkaloids. In 1971, only 7 alkaloids were known which possessed the common spirobenzylisoquinoline nucleus presented in formula. In 1980, a compendium of spirobenzylisoquinoline alkaloids appeared covering the physical and spectral data of 29 alkaloids. Since that time it has been demonstrated that 3 alkaloids—namely, fumarofine, fumaritrine, and fumaritridine—are indenobenzazepine alkaloids rather than spirobenzylisoquinolines. Additionally, 12 new spirobenzylisoquinoline alkaloids have been isolated from different plant sources including two N-norspirobenzylisoquinolines, a type of alkaloid which was previously unknown, one glycosidic alkaloid, parviflorine, and the first secospirobenzylisoquinoline alkaloid. Spirobenzylisoquinoline alkaloids have thus far been found only within the plant family Fumariaceae, and, more specifically, with few exceptions they occur within the genera Fumaria and Corydalis. A direct relationship is observed between the plant source and the oxygenation pattern of ring C of the spirobenzylisoquinolines. © 1990 Academic Press Inc.