The European Union has developed since the mid-1990's its own policy to prevent and fight against trafficking in human beings (THB). This phenomenon constitutes a threat to the internal security of the European Union, and it also entails severe violations of fundamental rights of its victims. While taking into account the evolution of the definition of THB, as well as the development of a EU multidisciplinary, integrated and holistic approach to THB, this doctoral research focuses on the external dimension of this policy. Firstly the analysis of the EU acquis in this field allows to determine the external competences at its disposal to promote the transposition of its standards beyond its borders. The second part focuses on the interactions between the European Union and the other actors active in this field. The objective is here to demonstrate that their interactions lead to the emergence of harmonized objectives and measures to combat THB in a comprehensive way. Finally the third part aims at assessing the implementation of European and international standards in national legal orders. Two case studies have been selected here: Belgium and Bosnia and Herzegovina.