This chapter presents an overview of the interactions between plant roots and root-feeding insect herbivores, focussing on changes in growth and physiology and crucially how roots are defended against insect attack. Several reviews have covered the ecology and management of insect root herbivores, together with their interactions with the abiotic and biotic soil environment. Therefore, the chapter focuses particularly on advances in our understanding of how plant mutualistic fungi may affect root-herbivores. This is an emerging area of research, with many attendant knowledge gaps, but we argue that this is an important component of how plants resist attack by belowground insect herbivores.