This thesis examines the theory of diplomacy in the formative phase of the European states-system. From the viewpoint of the discourse on the 'ideal ambassador', it explores early modern diplomacy as cultural history encompassing ideas, discourses, perceptions and 'codes'. The scope of study is the century following the Peace of Westphalia (1648), and three states and regions (France, Britain, the Holy Roman Empire) serve as case studies for a comparative approach of diplomatic theory and practice. In five parts, the adaptation of the theory and practice of diplomacy to the new demands of international relations after 1648 are considered. The first section sets the stage by illustrating that the mid-seventeenth century was regarded as a turning point in the practice of diplomacy. Part II examines diplomatic theory as a particular 'language' in its intellectual and socio-professional contexts. While published treatises on the 'ideal ambassador' build the core of this study, related genres of international law theory, ceremonial theory and political and state science are also taken into account. From the viewpoint of this diplomatic theory, the following section examines the ways in which the instruments and practices of diplomacy were aligned to the new framework. These ranged from changes in the structural framework of diplomacy to the evolution of norms and procedures of negotiation, international law and ceremonial. Part IV reconsiders the issue of 'professionalism' in diplomatic theory with regard to the preparation and training of diplomats. Special attention is given to proposals for diplomatic 'academies', which are for the first time examined in comparison. Finally, section V recasts the findings of this thesis in a comparative perspective. It underlines that, with the emergence of a states-system, the techniques of diplomacy became formalised and uniform, constituting a common European diplomatic practice. Against the background of the different regional and structural conditions, the alleged model role of France in the evolution of diplomatic theory and practice is re-evaluated.