In nature, plants interact with many organisms and need to integrate their responses to these diverse community members. Knowledge on plant-insect relationships has accumulated rapidly during the last decades. Yet, most studies on direct or indirect defences of plants against herbivory have treated herbivores as individual stressors. However, herbivores often consist of communities themselves, comprising organisms such as parasites and symbionts, that may have important effects on the herbivore phenotype, and consequently on interactions of the herbivore with its food plant. Here, we review how herbivore-associated organisms affect plant-herbivore interactions. Organisms associated with herbivores can directly affect how a plant interacts with their herbivorous hosts, by interfering with plant signal-transduction pathways, repressing the expression of plant defence-related genes, or altering plant secondary metabolism. In addition, herbivore-associated organisms can also affect plant responses indirectly by their effect on the behaviour and physiology of their herbivore host. The changes in plant phenotype that arise from herbivore-associated organisms may subsequently affect interactions with other community members, thereby impacting community dynamics. Furthermore, herbivore-associated organisms may act as a hidden driving force of plant-herbivore coevolution. Therefore, to understand plant-herbivore interactions it is important to realize that every single herbivorous insect constitutes a community in itself.