According to the PDE, there is a strict moral constraint against bringing about serious evil (harm) to an innocent person intentionally, but it is permissible in a wider range of circumstances to act in a way that brings about serious evil incidentally, as a foreseen but nonintended side effect. In Chapter 1, I discuss the key terms in this definition and I distinguish between an absolutist version of the PDE and a nonabsolutist one. I then introduce two principles that help guide agents when they foresee their conduct will cause incidental harm: the Principle of Proportionality and the Principle of Due Care. Finally, I describe the role of the PDE in just war theory and international humanitarian law, and I set out the plan for the remainder of the book.