Host or prey specificity is a highly desirable attribute of arthropod natural enemies imported for biological control of exotic pests. Therefore, whenever possible, candidates for biological control should be collected or reared from the target pest species or a closely related species. Natural enemies also should be collected from the native home or source of the pest invader, or from an ecologically homologous area. To help ensure a high affinity for a pest species, reasonable efforts should be made to determine the natural host or prey range of an enemy. Highly host- or prey-specific natural enemies tend to remain so but, occasionally, new associations are formed with different species. Host specificity is an important issue for those concerned about conservation of native species and natural habitats. This group includes many proponents of classical biological control because maintenance of global biodiversity increases the chances of finding new and potentially effective natural enemies.