Observations of the effects of living organisms on weeds dates from 1795 when an insect, Dactylopius ceylonicus, was introduced for drooping pricklypear (Opuntia vulgaris Miller) control over a vast area. Since that time, biological control of weeds employed mainly the classical strategy of introducing natural enemies from areas of co-evolution. Self-perpetuation and dissemination of these introduced enemies was essential to suppress successfully the weed below economic levels. This classical tactic is suited particularly for weeds that are distributed widely in less intensively cropped or noncropped areas. Guidelines to introduce foreign organisms for biological control of weeds in the United States have been established.