Bruce E. Tabashnik and Richard T. Roush Pesticide resistance is an increasingly urgent worldwide problem. Resistance to one or more pesticides has been documented in more than 440 species of insects and mites. Resistance in vectors of human dise8se, particularly malaria-transmit ting mosquitoes, is a serious threat to public health in many nations. Agricultural productivity is jeopardized because of widespread resistance in crop and livestock pests. Serious resistance problems are also evident in pests of the urban environ ment, most notably cockroaches. Better understanding of pesticide resistance is needed to devise techniques for managing resistance (Le. , slowing, preventing, or reversing development of resistance in pests and promoting it in beneficial natural enemies). At the same time, resistance is a dramatic example of evolution. Knowledge of resistance can thus provide fundamental insights into evolution, genetics, physiology, and ecology. Resistance management can help to reduce the harmful effects of pesticides by decreasing rates of pesticide use and prolonging the efficacy of environmentally safe pesticides. In response to resistance problems, the concentration or frequency of pesticide applications is often increased. Effective resistance management would reduce this type of increased pesticide use. Improved monitoring of resis tance would also decrease the number of ineffective pesticide applications that are made when a resistance problem exists but has not been diagnosed. Resistance often leads to replacement of one pesticide with another that is more expensive and less compatible with alternative controls.