© 2001 Elsevier Inc. All rights reserved. Parasitoids are a group of insects whose larvae develop on or in the bodies off other insects, which they eventually kill. The majority of parasitoids are wasps (Hymenoptera) or files (Diptera), and they are numerically very abundant and important in nearly all terrestrial ecosystems. The entry explores the diversity of life histories found in parasitoids, and the physiological and behavioral adaptations of endoparasitoids to developing inside their host. It also discusses their importance as model systems for investigating major issues in evolutionary theory, for example, the sex ratio. Parasitoids are important in population ecology because they regulate or control the populations of many of their hosts. The entry discusses their population biology and their use in agriculture and forestry as biological control agents.