This chapter discusses the occurrence and constitution of Senecio alkaloids. Considerable advances have been made in the isolation of new alkaloids, the determination of structures including absolute configurations, and the stereospecific synthesis of both bases and acids. It is now possible to get a glimpse at the overall picture and to write some general structures for acids and alkaloids. Structural studies have been considerably facilitated by nuclear magnetic resonance (NMR) spectra, mass spectra, and X-ray analysis, particularly where the hydrolysis of the alkaloid has resulted in extensive fragmentation. The chapter discusses the structure of the necines, necic acids, and alkaloids. Other pyrrolizidine alkaloids are also discussed in the chapter. The pyrrolizidine structure has more recently been found in a number of other alkaloids. © 1970, Elsevier Inc. All rights reserved.