Technical Abstract: Aphids are a group of piercing-sucking insects which engender significant economic losses on a variety of crops worldwide. The redoubtable impact of aphid injury on agriculture has been well-documented in the history of agronomy, dendrology, and horticulture, in particular, enology and pomology. The study of aphid-elicited injury is an interdisciplinary, if not a pandisciplinary topic of all agricultural sciences. The aphids are more than live plant-sap-syringes because they secrete and inject saliva into plants before ingesting plant sap. Plant responses to aphid feeding can be either asymptomatic or symptomatic in nature. Aphid-elicited injury symptoms on plants range from developmental desistance to neoplasm. Desistance is defined as the ceasing of plant growth (e.g., chlorosis and necrosis), while neoplasm is defined as abnormal proliferation of plant tissues (e.g., different types of tissue galls). We propose a classification scheme for aphid feeding injury in accordance with the symptoms that aphids elicit. Recent understandings of biochemical and physiological mechanisms of aphid injury symptom formation serve as the basis for the proposed classification scheme. Additionally, we discuss the ramifications of studying aphid injury in relation to improving crop resistance to aphid and other sap-feeding herbivores, as well as research frontiers in understanding etiology of aphid injury.