In the last thirty years the major classes of chemical insecticides (organochlorines, organophosphorus compounds, carbamates and pyrethroids) have been synthesised and developed into economically important chemicals for agricultural and other purposes. The course of development has moved the compounds in use from effective but persistent to highly effective but relatively non toxic and non persistent. The book opens with a review of currently available insecticides examined by class and use. The concept of pro-insecticides is then discussed; pro-insecticides are compounds which are converted into bioactivity by the metabolic activity of the target organism or host. By considering several older insecticides which act through bioactivation, rational design methods for suitable new compounds are suggested. Resistance to pyrethroids, and the mammalian toxicology of these compounds is discussed in detail. These subject matters preface consideration of the metabolism and toxicity of insecticides to fish. New areas for research and development which are attractive prospects for future work: chitin synthesis inhibitors and the vast range of natural chemicals which control insect behaviour; are reviewed and their potential explored.