The Peace of Westphalia is extensively described in modern historiography and regarded as a central event in the history of early modern international relations. However, the inherent logic of the preceding peace congress as the first international and multilateral delegated peace congress is much less explored. This research report delineates the main research developments and highlights current approaches as well as desiderata. Thereby the authors show that the lack of research on the peace congress as a diplomatic event results from its position between Historical Peace Research and New Diplomatic History. Originating from court research, New Diplomatic History has so far not paid much attention to the specifics of congress diplomacy. Accordingly, the authors stress the potential that lies in analysing the Westphalian Peace Congress from such a cultural historical perspective for Historical Peace Research, which is the only way allowing us to explore the difficulties of peacemaking in a profound and comprehensive manner.