Plant responses to herbivores are complex and depend on the type of feeding and the extent of tissue damage. Phloem-feeding insects like whiteflies and aphids, which cause minimal damage, trigger salicylic acid (SA)-dependent and jasmonic acid (JA)/ethylene-dependent signaling pathways. These pathways are similar to those used in pathogen defense. Other elicitors for defense may be more general. Chewing insects, such as caterpillars and beetles, cause more extensive damage and activate different pathways. Plants use both constitutive and induced defenses to protect against herbivores. Constitutive defenses include physical barriers and stored chemicals that deter herbivores. Induced defenses are activated upon herbivore attack and include direct defenses that interfere with feeding and indirect defenses through volatile compounds that attract predators. These responses help plants survive and reproduce. The balance between constitutive and induced defenses is crucial for plant survival. Most knowledge about plant responses to herbivores comes from studies on insects that damage foliage. Less is known about responses to herbivores that cause less damage, such as those that mine or gall. This review focuses on non-chewing herbivores. These herbivores activate several well-characterized defense and wound response pathways, as well as novel pathways. They produce elicitors that activate plant gene expression and volatile synthesis. The source of these elicitors is discussed. Piercing/sucking insects, such as aphids and whiteflies, have long interactions with plant cells and cause varying degrees of damage. Their feeding stylets pierce cells and consume fluids. Some herbivores, like thrips, are cell-content feeders that cause more extensive damage. The responses of plants to these herbivores are complex and involve multiple signaling pathways.Plant responses to herbivores are complex and depend on the type of feeding and the extent of tissue damage. Phloem-feeding insects like whiteflies and aphids, which cause minimal damage, trigger salicylic acid (SA)-dependent and jasmonic acid (JA)/ethylene-dependent signaling pathways. These pathways are similar to those used in pathogen defense. Other elicitors for defense may be more general. Chewing insects, such as caterpillars and beetles, cause more extensive damage and activate different pathways. Plants use both constitutive and induced defenses to protect against herbivores. Constitutive defenses include physical barriers and stored chemicals that deter herbivores. Induced defenses are activated upon herbivore attack and include direct defenses that interfere with feeding and indirect defenses through volatile compounds that attract predators. These responses help plants survive and reproduce. The balance between constitutive and induced defenses is crucial for plant survival. Most knowledge about plant responses to herbivores comes from studies on insects that damage foliage. Less is known about responses to herbivores that cause less damage, such as those that mine or gall. This review focuses on non-chewing herbivores. These herbivores activate several well-characterized defense and wound response pathways, as well as novel pathways. They produce elicitors that activate plant gene expression and volatile synthesis. The source of these elicitors is discussed. Piercing/sucking insects, such as aphids and whiteflies, have long interactions with plant cells and cause varying degrees of damage. Their feeding stylets pierce cells and consume fluids. Some herbivores, like thrips, are cell-content feeders that cause more extensive damage. The responses of plants to these herbivores are complex and involve multiple signaling pathways.