Cyanogenic glucosides are amino acid derived natural products widely distributed in the plant kingdom. Among the more than 3.000 plant species known to contain cyanogenic glucosides are important crop plants like sorghum, barley, cassava, clover, flax, lotus and almonds. The biosynthetic pathway involves two membrane bound cytochrome P450s and a soluble UDPG-glucosyltransferase that are assembled in a metabolon to secure “conveyor belt” synthesis of the cyanogenic glucoside without diffusion of toxic intermediates into the cytosol. Plants producing cyanogenic glucosides, also contain -glycosidases that upon cellular disruption of the plant tissue catalyze their conversion into keto compounds and hydrogen cyanide. This binary system - two sets of components which separately are chemically inert – provides the plants with an immediate chemical defence response to herbivores causing tissue damage. However, the trait of cyanogenesis is about 430 million years old enabling co-evolution of cyanogenic plants and their herbivores and pests. The great potential offered by transgenic plants to elucidate such interactions was illustrated by transfer of the entire pathway for synthesis of the cyanogenic glucoside dhurrin from sorghum to Arabidopsis using genetic engineering. The presence of dhurrin in transgenic Arabidopsis deterred feeding by Phyllotreta nemorum, thus unambiguously demonstrating that cyanogenic glucosides may confer resistance against selected herbivores. In contrast, the feeding behavior of Plutella xylostella did not change. Other insects like Zygaena filipendulae are able to sequester cyanogenic glucosides present in their host plant and to use the plant derived cyanogenic glucoside in their own defence. Z. filipendulae strives to achieve certain threshold contents and ratios of cyanogenic glucosides and is able to compensate for varying contents and different ratios of cyanogenic glucosides in the food plants by preferential de novo biosynthesis of the compound in demand. The defence system of the plant has thus been taken over by the insect. Accordingly, removal of a natural product from a plant does not necessarily increase its sensitivity towards important herbivores and detailed analyses are required to predict what happens with respect to plant-herbivore resistance when the pathway for cyanogenic glucoside synthesis is introduced into a new crop species or when the pathway is blocked in a cyanogenic crop plant like cassava.