The insecticides discussed in this chapter target structures and physiological systems unique to insects and similar creatures and are consequently of low mammalian toxicity. This is despite one group, the ecdysone agonists, targeting an insect steroid hormonal system. Curiously, a remarkably large number of these compounds, notwithstanding their structural diversity, have adverse effects on the mammalian haematological system. As these compounds target insect development, they are not â knock-downâ insecticides and are not very effective against established adult insect infestation.