The thesis deals with the question as to whether international organisations could be jointly responsible for violations of international law committed by peacekeepers deployed in a peacekeeping operation. The study starts by exploring the development of the concept of peacekeeping operations and the relations between the UN and four specific regional organisations (NATO, EU, AU and ECOWAS) on the basis of the applicable dispositions of the UN Charter. This examination as well as the following analysis of relations among these organisations, illustrate the evolution of a division of labour and an increase of cooperation between the UN and these organisations. It justifies the formulation of a presumption that international organisations could be jointly responsible as well as the formulation of a new criterion of attribution (normative control). The case-studies of specific peacekeeping operations confirm that in certain circumstances the UN and regional organisations have to be considered jointly responsible.