Plant defences against insect herbivores can be divided into 'static' or constitutive defences, and 'active' or induced defences, although the insecticidal compounds or proteins involved are often the same. Induced defences have aspects common to all plants, whereas the accumulation of constitutive defences is species-specific. Insect herbivores activate induced defences both locally and systemically by signalling pathways involving systemin, jasmonate, oligogalacturonic acid and hydrogen peroxide. Plants also respond to insect attack by producing volatiles, which can be used to deter herbivores, to communicate between parts of the plant, or between plants, to induce defence responses. Plant volatiles are also an important component in indirect defence. Herbivorous insects have adapted to tolerate plant defences, and such adaptations can also be constitutive or induced. Insects whose plant host range is limited are more likely to show constitutive adaptation to the insecticidal compounds they will encounter, whereas insects which feed on a wide range of plant species often use induced adaptations to overcome plant defences. Both plant defence and insect adaptation involve a metabolic cost, and in a natural system most plant-insect interactions involving herbivory reach a 'stand-off' where both host and herbivore survive but develop suboptimally. Contents Summary 145 I. Introduction 146 II. Accumulation of defensive compounds and induced resistance 146 III. Signalling pathways in wound-induced resistance 147 IV. Insect modulation of the wounding response 155 V. Insects which evade the wounding response 156 VI. Insect-induced emission of volatiles and tritrophic interactions 157 VII. Insect adaptation to plant defences 160 Conclusions 163 Acknowlegements 163 References 163.