Many environmental factors may affect herbivore/ plant interactions by increasing the level of resistance or susceptibility of the plant to the herbivore. Among the factors with a demonstrated capability to induce changes in levels of resistance are temperature, solar radiation, water stress, soil fertility, insecticides, herbicides, fungicides, growth regulators, pathogen infection, weed competition, and previous or concurrent herbivore attack. Most chemical factors, responsible for the resistance of plants to arthropods studied to date, have involved the genetically controlled, injury-independent accumulation of metabolites with allomonal activity.A completely different plant defense strategy has been demonstrated for many pathogens for which phytoalexin accumulation is the result of a post-challenge response.The aforementioned environmental factors, including pestrelated injury, can induce de novo synthesis and accumulation of compounds with allomonal properties (phytoalexins) or change the relative concentration of both nutrient and non-nutrient compounds.The focus in this paper is on the post-infestation induction of resistance by arthropods, in a manner up until recently known to result only from pathogen infection. The insect-resistance role of phytoalexins is discussed and reference is made to other possible natural sources of inducers of resistance.Antiherbivory in plants has been ascribed mainly to the presence of physical defenses or to the injury-independent accumulation of secondary metabolites that have allomonal