Plants produce compounds which can act as semiochemicals, i.e. as signals modifying the behaviour and development of other organisms without having direct physiological activity, and which generally have extremely benign toxicological properties. Such semiochemicals include plant stress signals and possibly also phytopheromones associated with the induction of defence systems but which may also be exploited by colonizing organisms. Since it is now possible to use sophisticated electrophysiological techniques to investigate insect interactions with phytopheromones, the detection and characterization of such signalling systems is greatly facilitated, particularly when combined with behavioural techniques for confirming the biological relevance of the signals. As well as the contribution of these studies to the understanding of plant-insect and plant-plant interactions, plant signals are potentially valuable in the regulation of gene expression for improved or alternative approaches to crop protection. It would also be possible to use these signals to 'switch on', when required, other genes associated with beneficial agronomic or nutritional crop traits.