Classical biological control of invasive tephritid fruit fly pests using parasitoids has been practiced for over one century in Hawaii, and has resulted in the introduction of over 30 hymenopteran parasitoids from Africa, Australia and South Asia. Establishment of several effective opiine parasitoids has greatly suppressed populations of two major fruit fly pests, becoming the most successful example of classical biological control of tephritid pests in the world. However, several issues, including competitive displacement or interference among the introduced parasitoids, and potential non-target impacts have been raised recently. In this study we addressed (1) the current status of classical biological control of tephritid pests using parasitoids in Hawaii; (2) the ecological mechanisms underlying competitive displacement among the introduced parasitoids in Hawaii, in particular the competitive superiority of early acting species; and (3) potential non-target impacts as a result of competition-mediated host expansion or facultative hyperparasitism. Finally, we discuss the significance of interactions among fruit fly parasitoids in relation to future biological control of fruit fly pests.