The symbiotic relationship between terrestrial plants and arbuscular mycorrhizal (AM) fungi is a key driver of plant nutritional and defence traits influencing insect herbivory. These tripartite interactions have been fundamental to shaping the evolution of land plants and the diversity of insect herbivores. Surprisingly, we have little understanding of how these interactions will function under elevated atmospheric CO 2 concentrations (eCO 2 ), despite the considerable implications for both natural and managed ecosystems. Although substantial research has revealed how eCO 2 alters mycorrhizal–plant interactions, or plant–herbivore interactions, there is a stark scarcity of studies which investigate how eCO 2 impacts mycorrhizal-mediated plant–insect herbivore relationships. Here, we synthesise some of the main effects of eCO 2 on the mycorrhizal symbiosis, the concomitant impacts on plant nutrient dynamics and secondary metabolism, and how eCO 2 -driven changes in plant growth, biochemistry and communities impact insect herbivores. We point out that potential mechanistic drivers of AM fungal–plant–insect herbivore relationships under eCO 2 can function antagonistically and are highly context-dependent, which poses a particular challenge. Still, we hypothesise as to the potential outcomes for AM fungal–plant–herbivore dynamics under eCO 2 . We identify key research priorities to tackle the substantial gap in our understanding. If ecological theory is to effectively inform agricultural and natural management practices in the future, research needs to directly investigate how changes in global atmospheric CO 2 concentrations impact the tripartite relationship between AM fungi, plants and insect herbivores. A plain language summary is available for this article.