The principle of double effect (PDE) is an important principle of medical ethics. Yet it is often poorly understood, abused, and criticized. In this article, the author explains the purpose of the PDE. It is further explained that the PDE requires (1) that there be two effects that follow from the doctor's actions; (2) that one effect be intended and the other unintended; (3) that the unintended bad effect cannot be the cause of the intended good effect; and (4) that the good effect must outweigh the bad effect. The differences between intentions, beliefs, desires, and motives are explained. Screening questions about clinical intentions are proposed, and guidelines for morphine dosing that would be consistent with an intention to relieve pain and not to cause death are offered. The PDE does not relieve physicians of responsibility for their actions, but only sets conditions under which they may not be held morally culpable for the unintended side effects of actions they undertake responsibly. As an example, the use of morphine is distinguished from the use of pancuronium to treat the gasping of a patient for whom ventilator treatment is being discontinued. The PDE would permit the use of morphine, but the PDE would not permit the use of pancuronium in such a case