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. These plants also contain degradative enzymes that upon cellular disruption of the plant tissue are brought in contact with the cyanogenic glucosides causing hydrogen cyanide release. This binary system - two sets of components which separately are chemically inert – provides such plants with an immediate chemical defense response to herbivores and pathogens 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. In some cases, insects are able to sequester cyanogenic glucosides present in their host plant and to use the plant derived cyanogenic glucoside in their own defense. In some instances, where insects are not able to obtain the desired amount of cyanogenic glucosides from their host plants, they are able to carry out de novo synthesis. The defense system of the plant has thus been taken over by the insect. Likewise, some fungal pathogens appear to utilize cyanogenic glucosides as host recognition factors so that removal of the compounds increases fungal resistance. Accordingly, it is not obvious to predict what happens with respect to plant-herbivore and pest 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. We have studied these intimate relationships using genetic engineering and the results obtained with barley, cassava, lotus and Arabidopsis will be presented