Parasitoids constitute an important component of the biodiversity of terrestrial ecosystems and have become a recent focus for investigations of pattern and process in ecological communities. This article uses parasitoid guilds to characterize the structure of parasitoid communities associated with endopterygote hosts. Parasitoid guilds are defined by three parameters, the host stage attacked, the host stage killed, and the mode of parasitism (internal or external development). A series of 12 guilds are recognized from the patterns of host exploitation by parasitoids; one that utilizes the host egg stage as a resource for parasitoid development, three that utilize the host larva, four that utilize the host prepupa, two that utilize the host pupa, and two that utilize the host adult. A comparison of the parasitoid guilds across endopterygote host orders illustrates the taxonomic diversity of parasitoids in each guild and provides an indication of some of the more frequent parasitoid taxa associated with each guild. The value of the parasitoid guild as an ecological unit that both classifies the functional ecology of parasitoid species and the structure of parasitoid communities is discussed.