The development of pesticide resistance in arthropod pests, plant pathogens and weeds can be viewed and studied from two contrasting perspectives. At a fundamental level, resistance provides an almost ideal example of adaptation to withstand severe environmental stress. Population geneticists, biochemists and, most recently, molecular biologists have cast considerable light on the nature of this adaptation in diverse taxonomic groups, and on factors determining its selection and spread within and between populations. Unlike most evolutionary phenomena, however, resistance is also of immediate...