KIE: In one of a series of articles on philosophical medical ethics, Gillon examines the Roman Catholic doctrine of double effect as it applies to discussions of the moral difference between acts and omissions in patient care. The doctrine holds that, in the context of actions that have both good and bad effects, an action that has a bad effect is morally permissible if (a) the action itself is good, (b) its perpetrator's intention is solely to produce the good effect, (c) the good effect is not achieved through the bad, and (d) there is sufficient reason to permit the bad effect. Gillon concludes that, while the doctrine of double effect is unlikely to be accepted fully by non-absolutists, some of its claims are useful in deciding which clinical interventions are morally justified.