Induced defence responses are elicited when plants are exposed to biotic stresses such as attack by herbivores or pathogens. In nature, plants are often subjected to attack by more than one organism, and defence responses elicited by one organism can thereby be modified by the presence of another. Below-ground attack can influence responses to above-ground attack and vice versa, due to systemic induction of defence metabolism. In some interactions defence is enhanced through prior attack by another organism, whereas in others there are conflicting signals. Recent research has shown how plants integrate these signals to coordinate defence by regulation of key metabolic pathways, although there is still much to be learnt.