The biologic activity of coumarin (91645) compounds is discussed. Uses of these substances in food, perfumes, and cosmetics is reviewed, and data on metabolic pathways is summarized. The results of numerous investigations on the toxic effects of coumarin, hydroxycoumarins, 4-hydroxycoumarin (1076386), and psoralens are discussed, along with studies of the formation and concentration of these compounds in mold and plant tissues. Biochemical and metabolic theories of coumarin formation in molds are evaluated. The author suggests that a symbiotic relationship exists between plant tissue and coumarin producing molds. These compounds appear to have a great potential for beneficial use; however, a better understanding of their mechanisms of action is needed.