Pyrethrins are natural products produced by various species of Chrysanthemums. Their insecticidal activity was known as early as the beginning of the eighteenth century, and they were used worldwide by 1850. 1 Pyrethrum is the product prepared from ground flowers by extraction with organic solvents, and it consists of a mixture of pyrethrins. The concentrate typically contains 90 to 100% pyrethrins. Pyrethrum is essentially nontoxic to mammals but is an effective, rapid acting insecticide. Prior to the development of DDT and other chlorinated hydrocarbon insecticides (CHis), pyrethrum was extensively used for both agricultural and domestic purposes. Today it is only used as a domestic insecticide. Its other uses were rapidly taken over by CHis for two reasons. Although effective in relatively low concentrations, pyrethrins break down rapidly in light. This rapid breakdown and resulting high costs from continual reapplication led to a preference for the more persistent CHis. However, the high selectivity of pyrethrum for insects and its low mammalian toxicity led to a search for synthetic derivatives possessing more stable chemical properties. The pyrethroids are the result. Since the 1970s, hundreds of synthetic pyrethroids have been made, of which six to ten see widespread use today. The modem pyrethroids are potent, selective insecticides. For example, deltamethrin, gram for gram, is 2700 to 5560 times more toxic to houseflies than to rats. 2 Today the pyrethroids comprise a large part of the world foliar market. In spite of their widespread use, reports of human poisoning by pyrethroids are uncommon. A number of excellent reviews on pyrethroids have appeared recently, 2·' and this chapter will emphasize mechanisms of action and human toxicity.