A comparative analysis of three natural enemy guilds is used to illustrate a conceptual framework for guild analysis and to derive predictions for applied biological control. The guilds consist of: a natural, coevolved guild of parasites (seven species) associated with a native cecidomyiid midge,Rhopalomyia californicaFelt, which develops in galls onBaccharis pilularisDC in northern California; a diverse parasite guild (12 species), believed to have been restructured by the addition of an exotic species, associated with a native diaspidid,Melanaspis obscura(Comstock) (obscure scale), on pecan in southern Texas; and a synthetic or anthropogenic guild of phytoseiid mites (four commercially available species) released against twospotted spider mite,Tetranychus urticaeKoch, on bean plants in northern California. These guilds are analyzed according to the following framework: ecological-evolutionary history, empirical patterns in space and time (e.g., spatial distribution of species per host patch), factors affecting guild structure (e.g., interspecific competition, structure of host population), ecological effect on the host population (e.g., spatial density dependence), and relationship between guild structure and ecological effect (e.g., species-dependent mortality). It is suggested that analysis of natural enemy guilds may be of value in deriving introduction strategies in both classical and augmentative biological control. The importation ofEncarsia aurantii(Howard) into California for control of obscure scale is described as a case in point. Some additional implications of guild analysis for biological control are discussed.