Introduction. Necessity and proportionality are broad principles that apply to a variety of different settings under international law, including the law of self-defence. This seems undisputed today, even though Article 51 of the Charter of the United Nations (UN Charter) does not mention ‘proportionality/proportionate’ at all, and ‘necessity/necessary’ only in relation to the Security Council. Yet Article 51 UN Charter does not regulate the right to self-defence comprehensively; in order for it to be applicable, ‘external factors’ need to be taken into account. That necessity and proportionality are two such factors is generally agreed. Pursuant to the jurisprudence of the ICJ ‘[t]he submission of the exercise of the right of self-defence to the conditions of necessity and proportionality is a rule of customary international law’. In this respect, the Court was surely correct to note that ‘[t]he conditions for the exercise of the right of self-defence are well settled’. ‘Ritual incantations’ of necessity and proportionality, however, do not solve problems of application. These appear at two levels. Predictably, when measuring specific acts of self-defence against the yardsticks of necessity and proportionality, lawyers are likely to reach different results. In so far as this reflects different assessments of the facts, this may be in the nature of things. There is however a second problem: necessity and proportionality are ‘consistently referred to … but rarely, if ever, analysed in relation to the Charter scheme on self-defence’. As a consequence, there is much uncertainty about the precise content of the ‘necessity test’ and the ‘proportionality equation’, and about their interrelation.