The dithiophenes and trithiophenes reviewed in this article are part of a large group of biogenetically related molecules found in plants of the family Compo sitae (Asteraceae). They include compounds having a variable number of unsaturations, particularly double bonds and triple bonds, which occur singly or in combinations. The first report of a naturally occurring trithiophene, ex-terthienyl in the flowers of Tagetes erecta, appeared in 1947 (270); the first naturally occurring dithiophene was isolated from Bidens radiata and described in 1961 (142). The book Naturally Occurring Acetylenes, by BOHLMANN et al. (29), is a superb review of the field up to 1972. Interestingly, out of its more than 500 pages, only two were devoted to physiological and pharmacological aspects. BOHLMANN and ZDERO later contributed one chapter, Naturally Occurring Thiophenes , to a volume Thiophenes and its Derivatives which appeared in 1985 (57). This chapter presents a survey of such thiophenes based on biogenetic considerations, includes an extensive analysis of the distribution of these compounds, and discusses methods of analysis based on UV, lH-NMR, 13C-NMR, and mass spectra. The most recent references in the chapter came from publications appearing in 1981. In view of the information on naturally occurring dithiophenes and trithiophenes already available from these sources, this survey emphas- izes the occurrence, the biosynthesis, the synthesis, the photophysical and photochemical properties, and especially the biological properties of these molecules. II.