Many studies of plant resistance have focused on plant responses to pathogens or chewing insects, but few have addressed the response to piercing-sucking insects. Chewing insects, such as the beet armyworm, cause extensive mechanical damage to the plant, and induce many defense-related genes, including some genes that overlap with the plant’s response to mechanical wounding. Piercing-sucking insects, such as aphids, cause much less mechanical damage, and have been shown to induce multiple signaling pathways, including those induced by chewing insects and those induced by pathogens such as viruses, bacteria, or fungi. In this study, tomato (Lycopersicon esculentum) was used to identify genes that are differentially expressed when challenged with insects from two distinct feeding guilds: piercing-sucking insects (the potato aphid, (Macrosiphum euphorbiae), and chewing insects (the beet armyworm, (Spodoptera exigua). Tissue samples were collected at 12 and 24 hours, and control plants (no insect challenge) were compared to samples challenged with either aphids or armyworms using the TOM1 cDNA microarray (Boyce Thompson Institute, Cornell University), which represents approximately 25% of the tomato genome and includes over 400 defense-related genes. This experiment identified genes from multiple functional classes in tomato that were either up- or down-regulated in response to insect feeding, and that differed in response to aphids versus armyworms.