1. Synthetic pyrethroids, based on the naturally-occurring insecticidal components of pyrethrum extract, emerged in the 1970s as the fourth major chemical class of synthetic insecticides. They are widely used today in the control of agriculture and household pests and disease vectors. 2. Early efforts in the design of synthetic analogues focused on the need to identify novel structural moieties that preserved or enhanced intrinsic insecticidal activity while eliminating known sites of metabolic and photolytic attack in the natural compounds. Subsequent efforts focused on achieving high levels of insecticidal activity while minimizing costs of synthesis and retaining desirable levels of selective toxicity. 3. The synthetic compounds obtained in these efforts constitute a group of insecticides having unprecedented biological activity against target species with low acute toxicity to mammals. 4. The evolutionary development of the pyrethroids illustrates how knowledge of metabolic fate can contribute to the design of novel insecticides with improved insecticidal activity and selective toxicity.