Interactions between plants and herbivores are central to all ecosystems. Although numerous types of interaction have evolved, there is considerable overlap in recognition, signal transduction and gene expression events that orchestrate the plant defence. Responses to herbivore damage can be triggered by simple wounding or insect-derived elicitors such as certain enzymes, fatty acid-derived conjugates, other low-molecular-weight aliphatic compounds and peptides generated from degradation of ingested plant material. Early induced responses in host plants are characterised by membrane depolarisation, intracellular [Ca²⁺] transients and reactive oxygen species production, followed by the activation of protein kinases and a downstream phytohormone networks that coordinate particular responses. Induced and constitutive toxic or harmful secondary metabolites are crucial components of host defence. Also, proteinase inhibitors or threonine deaminase can reduce the nutritional value of the plant tissue by interfering with the herbivore digestion. Synergistic effects between plant constituents ensure efficient mobilisation of defences that cannot be bypassed easily. Emissions of induced volatile compounds influence local and long-range interactions by repelling herbivores and attracting parasites and parasitoids from a distance, thus employing a third trophic level. Moreover, uninfested parts of the same and neighbouring plants can respond to volatiles by upregulating defence-related genes (priming), allowing an accelerated response to actual damage and thereby reducing the metabolic costs that are associated with a fully operational defence system.