This article attempts to demonstrate that the law of self-defence in the UN-Charter is uncertain. Nobody doubts that the Charter permits self-defence, but the precise modalities of the right are unclear. This is not an exposé of the whole spectrum of issues that can arise or have arisen. Rather, the article considers a few examples and analyses the structure of the arguments scholars and tribunals employ in order to find out where our limits of ‘legal cognition’ lie. Several topics are examined, inter alia, whether an ‘armed attack’ is a necessary condition for the exercise of self-defence under the UN-Charter; the actions qualifying as armed attack; whether an armed attack has to occur or whether the victim state merely reasonably has to expect it to occur; and whether individuals can commit an armed attack. It will become clear throughout the article that the closer one looks at the law of self-defence, the less certain the law becomes. I submit that the uncertainty of the law on self-defence is caused by a lack of legal regulation – there is only so much that Article 51 can tell us. Scholars importing their own ideas of what scope self-defence should have do not enhance our understanding of the positive law, their designs simply are not law.