The activity of gall-inducing aphids causes multifaceted alterations in host plant tissues leading to the formation of distinct gall structures. This chapter explores the intricate interactions between aphids and their host plants, focusing on the roles of biomolecules and signalling pathways in gall formation and development. The roles of plant hormones, enzymes, reactive oxygen species, primary metabolites, and phenolics in facilitating nutrient mobilization, structural modifications, and stress mitigation within gall tissues are discusses in detail. This chapter also highlights the need for expanded research efforts, particularly focused on the molecular mechanisms governing aphids’ gall development across different stages.