The term pesticides refers to a large body of diverse chemicals that includes insecticides, herbicides, fungicides, rodenticides, and fumigants employed to control one or more species deemed to be undesirable from the human viewpoint. Pesticides are of environmental concern for two main reasons. Although considerable progress has been made with respect to their selective toxicity, many still possess signicant toxicity for humans, and many are persistent poisons, so that their long biological T1/2 allows bioaccumulation and biomagnication up the food chain (see Chapter 3). There is, thus, the possibility that they may enter human food supplies as well as constituting an ecological hazard. By their very nature, pesticides must have an impact on any ecosystem since they are designed to modify it by their selective elimination of certain species. As is always the case in considering chemicals used in the service of humankind, there is a complex risk-benet equation that must be taken into account in making decisions regarding the use of pesticides. There is no question that they have increased agricultural production when used properly, and they have, in the past, been highly effective in controlling the insect vectors of human diseases like malaria and yellow fever spread by mosquitoes, and African sleeping sickness, which affects both humans and animals and which is spread by the tsetse y. As shall be seen, however, these gains have not been without their problems.