To understand trophic interactions within food webs, we need to be able to predict the nutritional consequences for a herbivore of ingesting a particular plant; however, grinding up plant tissues and measuring their chemical composition does not provide an indication of the ‘quality’ of that plant to herbivores. The ability of an insect herbivore that chews plant leaves to absorb nutrients depends on how these nutrients have been ‘packaged’ within the plant interacting with the insect’s morphological tools (typically the mandibles and gastrointestinal tract), and its behavioural and physiological flexibility. Interactions between plants and their herbivores sit at the nexus of all food webs, and how well an animal matches its demand for nutrients with supply from the environment affects not only that individual, but has population- and community-level consequences. Plant defence theory has typically been approached from a plant’s perspective, without taking into account the fixed or plastic mechanisms herbivores have to exploit host plants. There is an emerging understanding that many species interactions are context-dependent, and recent evidence suggests that the probability of a leaf being eaten depends on a complex suite of plant and herbivore traits. The next step towards an integrated understanding of plant–herbivore dynamics requires insights into the ability of the herbivore to gain the nutrients required from plants.