Computer network attacks are becoming more common as means for attacking states or private entities. The Stuxnet worm is a recent example of this kind of attack and shows the actuality of the increasing problem. This paper covers the questions of what the concepts of use of force and armed attack are generally considered to comprise, in international law, and the arguments in favour of or against applicability of the former concepts to computer network attacks. Computer network attacks constitute a new type of warfare, which poses new questions for the international community to handle. The main focus of this paper is when computer network attacks are used by one state to attack another state and how this may be handled in international law on resort to force, jus ad bellum. The discussion whether international law can cover computer network attacks between states is becoming more important and urgent as technology concerning computer network attacks is developing at high speed.