Transnational law enforcement operations are on the increase. The current counter-drug operations in Afghanistan, the fight against piracy in the Gulf of Aden and law enforcement activities in the Democratic Republic of the Congo by UN peacekeepers are exemplary of this trend. In 2012, the United Nations Security Council - for the first time in its history - qualified transnational organized crime phenomena as 'serious threats to international peace' and thus paved the way for future transnational law enforcement operations on the basis of the UN-Charter.Yet, in spite of a rapidly growing corpus of international practice, transnational law enforcement operations remain poorly understood and insufficiently regulated, often to the detriment of affected communities and the long-term success of sustainable crime/threat reduction.Against this backdrop, the proposed research project traces and analyses the newly emerging legal regime of 'transnational security law', i.e. the rules applicable to transnational law enforcement operations. The research project takes stock of contemporary international (state) practice and analyses how the international legal framework is currently adapting to this new type of operation. On this basis the research project aims to deduce a set of legal principles governing transnational law enforcement operations with a view to ensuring their legitimacy and long-term effectiveness. Thereby, the proposed research project will make a significant and indeed foundational contribution to the newly emerging legal discipline of 'transnational security law'.