The majority of studies on insect resistance have focused on plant defenses against chewing insects, such as caterpillars. Far less is known about resistance to piercing-sucking insects, such as aphids. We are currently comparing transcript profiles induced in tomato by aphid versus caterpillar feeding, in order to understand plants’ differential responses to these two feeding guilds. We are also using tomato as a model system to investigate acquired and innate resistance against the potato aphid, Macrosiphum euphorbiae. Acquired resistance depends upon broad-spectrum systemic defenses that are induced by an initial pest infestation, and that render plants less susceptible to subsequent attack. Innate resistance, in contrast, targets specific pests, and limits their initial establishment on the plant. In tomato, innate resistance to the potato aphid depends upon a resistance (R) gene called Mi-1.2. We are currently using cDNA microarrays to characterize gene expression profiles induced by aphid feeding in tomato lines with and without Mi-1.2, in order to investigate the mode of action of this gene. We are also investigating broad-spectrum defenses that may reduce potato aphid proliferation on susceptible tomato cultivars. We have found that aphid population growth is reduced by artificial induction of two divergent forms of acquired resistance: jasmonate-dependent defenses, which are induced by chewing insects, and salicylate-dependent defenses, which are typically associated with pathogen attack. We are currently using mutant tomato lines to investigate the role of these defensive pathways in limiting aphid infestation on tomato.