Plants can respond actively to damage by herbivores. In addition to a mode of defence that is directly aimed at the herbivore itself, plants can emit volatiles that attract carnivores, i.e. the enemies of their enemies. Knowledge of the mechanisms underlying the induction of these herbivore-induced plant volatiles and of the responses of the carnivores is progressing rapidly. Inferences on the initial causes of evolution of herbivore-induced plant volatiles remain conjectural. However, once plant–carnivore interactions have evolved to the net benefit of both participants this mutualism is expected to have evolutionary and ecological consequences for the three trophic levels involved. When plant selection and foraging behaviour of natural enemies is linked to plant fitness this can influence different aspects of the plant defence strategy. The way carnivores perceive and process plant information may influence the evolution of the plant signal (i.e. quantitative and qualitative composition of the odour blend in response to herbivore damage). Vice versa, the signal-to-noise ratio of the information may influence the way carnivores respond to plant cues (innately or through learning). Selection will act on herbivores to disconnect the plant-carnivore link, for example by boycotting the informational value of herbivore-induced synomones. Through plant selection and feeding behaviour herbivores can influence their chance of being found by carnivores. Hence, responses of carnivores to plant cues can influence the evolution of food-plant use by herbivores. The conspiracy between plants and carnivores is at the heart of evolutionary ecology, and wide open for experimental and theoretical investigations.