Several early biological control introductions, mostly of vertebrates and generally unsupervised by biological control experts, have led to unintended inimical effects on species in pristine habitats. A few such examples are known for biological control insects. However, particularly regarding the effects of entomophages, it is likely that biological control introductions have produced other unintended, harmful effects, but these are largely unknown. Subtle indirect ecological effects and the potential of natural enemies for ecological shift and evolutionary change argue for more stringent regulation and prerelease studies, although no protocol will be foolproof. Assessment of potential costs and benefits of a biological control project, though important, are beclouded by the unsatisfactory nature of existing methods of assigning costs to events of conservation concern, such as the disappearance of a rare species of no immediate economic benefit to humans.