Plants provide humans with oxygen, food, fibre and fuel, but their effectiveness in performing these roles is affected by herbivores. Historically, studies on insect herbivory have primarily addressed pest outbreaks, which have indisputable ecological and economic consequences. By contrast, less attention has been paid to background (‘normal’) insect herbivory (BIH), which inflicts minor damage but acts on plants continuously. In this review, we introduce BIH as a phenomenon of great ecological and evolutionary importance, summarize the current knowledge regarding the levels and patterns of BIH and the effects of BIH on individual plants, plant communities and ecosystem-level processes, and discuss the methodology of studies addressing BIH. In the long term, global terrestrial net primary production (NPP) is more strongly affected by BIH than by the outbreaks of eruptive insect species. Plant responses to BIH differ from their responses to severe damage, and abiotic drivers of global change may have different effects on background versus outbreak herbivory. Minor changes in BIH caused by human activities may have profound but imperfectly understood consequences for the structure and functions of terrestrial ecosystems. This justifies the urgent need to move the focus away from rare bouts of severe plant damage by insects to the ubiquitous phenomenon of BIH in ecosystem-level studies and away from episodic major damage to chronic minor damage in studies of plant–herbivore interactions.