Aphids are serious pests of agricultural and ornamental plants and important model systems for hemipteran–plant interactions. The long evolutionary history of aphids with their host plants has resulted in a variety of systems that provide insight into the different adaptation strategies of aphids to plants and vice versa. In the past, various plant–aphid interactions have been documented, but lack of functional tools has limited molecular studies on the mechanisms of plant–aphid interactions. Recent technological advances have begun to reveal plant–aphid interactions at the molecular level and to increase our knowledge of the mechanisms of aphid adaptation or specialization to different host plants. In this article, we compile and analyze available information on plant–aphid interactions, discuss the limitations of current knowledge, and argue for new research directions. We advocate for more work that takes advantage of natural systems and recently established molecular techniques to obtain a comprehensive view of plant–aphid interaction mechanisms.