The Doctrine of Double Effect (DDE) is an influential non-consequentialist principle positing a role for intention in affecting the moral permissibility of some actions. In particular, the DDE focuses on the intend/foresee distinction, the core claim being that it is sometimes permissible to bring about as a foreseen but unintended side-effect of one’s action some harm it would have been impermissible to aim at as a means or as an end, all else being equal. This article explores the meaning and application of the DDE along with current debates over the nature of the intend/foresee distinction and its moral significance. How is the line between intended effects and merely foreseen but unintended side-effects to be drawn, and how are problem cases best handled? What is an appropriate methodology for debating the tenability of the DDE? How might the DDE interact with other non-consequentialist principles, and how might it be modified to capture other related factors (resulting perhaps in a Doctrine of Triple Effect) or to avoid certain problem cases? Does the DDE make permissibility turn on the actual intentions of particular agents, as critics such as Thomson and Scanlon assume, or is the role of intention more abstract?