International law is said to be a distinct profession with institutions and journals first in the 1870's. Nevertheless, from the Vienna Congress (1815) to the Franco-Prussian Wars (1870-1871), lawyers have initiated professional practices that related to the development of International Law. They were involved in foreign offices, scientific academies, and universities, they wrote textbooks and articles and formed networks. This project aims to investigate the interaction between foreign offices and international lawyers as well as the link between political migration of lawyers and their implication in the making of international law. This research will therefore shed light on the discourses and processes leading to the institutionalisation of International Law. For the first time, it will also analyse closely the interactions between foreign offices and International Law as well as the juridification of international affairs in the 19th century. To do so, this project will benefit from the use of unpublished primary sources coming for the foreign offices of France, Prussia (Germany), Austria, Belgium, Great-Britain and Russia.