Modern society is characterized by the growth of new, technologically manufactured risks which follow human progress and innovation. Nuclear technology is an example of such human-induced sources of risks. Nuclear risk or the risk of radioactive contamination is a transboundary risk in the sense that it crosses the boundaries of nation-states uninvited, being transported by air, wind and water. This thesis explores how nation-states cooperate to manage transboundary nuclear risks. The focus is on Norway and the efforts made by Norwegian authorities since the early 1990’s to reduce transboundary nuclear risk stemming from nuclear technologies and activities in Northwest Russia. The thesis aims to explore what approach or combination of approaches a state can use during its cooperation with other states in order to successfully manage and reduce transboundary nuclear risk. The role of non-governmental and intergovernmental organizations in international cooperation on nuclear risk reduction is also analyzed. My findings show that in order to reduce transboundary nuclear risk a state can apply three support approaches both in regional cooperation with the neighbouring states, but also on a more global level, involving international organizations and other states. First, by using the approach of collective learning, a state can gather information and knowledge on the risk at stake and thus define what needs to be done. Second, a state can provide technical and financial assistance to the state which is the risk manufacturer. However, to control that the assistance is used as it is supposed to, the risk of implementation failure must be addressed as well. Thus the compliance control approach is the third approach that needs to be applied. When it comes to international organizations, the thesis shows that both NGOs and IGOs can serve as forums for collective learning and creation of consensual knowledge on the risk at stake. However, while IGOs, just as nation-states, can provide technical/financial assistance to the risk source country, NGOs are more suitable to serve as creative mediators during negotiations of intergovernmental agreements on nuclear risk reduction. Nevertheless, it is not sure that the combination of these support approaches is enough to address the risk of ineffective abatement: the possibility that all the measures undertaken still will not lead to successful risk reduction, even when there is no implementation failure.