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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 practical and economic significance. Not only has the number of resistant species continued to increase inexorably, but there has been an alarming increase in the severity and extent of some resistance problems. Cases of organisms resisting virtually all available pesticides are by no means uncommon, and pose a formidable challenge in view of present difficulties in discovering and developing novel chemicals. Although most occurrences of resistance were initially monofactorial, resistance now frequently involves a suite of coexisting mechanisms that protect organisms against the same or different pesticide groups, and may even predispose them to resist new, as yet unused chemicals.