Cannibalism is widespread among insects and can be important in regulating population size. Its incidence among predators has received much attention, although it also is common in many phytophagous insects. Our review of the primary literature reveals that the probability of cannibalism in phytophagous insects is strongly influenced by feeding guild. Insects in enclosed feeding niches, such as seeds, fruits, stored products, stems, and trunks, are often highly cannibalistic. Multiple factors can influence the propensity to cannibalize, including: density, limitations in the amount or quality of food, developmental asynchrony, age, parasitism, high temperature or humidity. Cannibalism may impart a fitness advantage by improving nutrition, thereby enhancing longevity, developmental rate, and reproduction, and also increasing levels of defensive compounds that are acquired from the victim. Cannibalism may serve to regulate population size and suppress population outbreak, stabilize host plant-insect relationships, and reduce parasitism rates. In some species, however, cannibalism may have negative fitness consequences, such as horizontal transmission of parasites or pathogens. Finally, the evolution of cannibalism may be constrained by dietary specialization that accompanies adaptation to phytophagy.