This article analyses the role and content of proportionality under contemporary international law governing the use of force, with a view to clarifying the legal framework governing the conduct of the parties to an armed conflict. In the system of jus ad bellum, protection is primarily granted to the interest of the attacked state in repelling the attack; the other competing interests are considered only to curtail the choice of the means to be employed in order to achieve that aim. Conversely, in the system of jus in bello there is by definition no prevailing interest, but instead a variety of interests and values which are entitled to equal protection of the law and must be balanced against each other. The existence of two distinct normative systems, with distinct standards of legality applicable to the same conduct, does not as a rule give rise to major problems. The legality of recourse to force is measured against the proportionality of self-defence, whereas individual actions would have to conform to the requirement of proportionality in jus in bello. However, beyond the large area in which these two standards overlap, there might be situations in which the strict application of the jus ad bellum standard makes it impossible to achieve the aims of jus in bello. In these cases, the proportionality test under jus in bello must be regarded as part of the proportionality test under jus ad bellum. States must thus take humanitarian implications into account in determining the level of security they may seek to obtain using military action.