This chapter outlines a history of treatises and other writings on the ambassador in early modern Europe. After providing some background on late medieval legal literature on the topic, the chapter explores early modern literature, from the 15th to the early 18th century, and focuses on some particular issues. The extent to which treatises and other writings on the ambassador can be considered to constitute a specific literary genre is be questioned; the different forms this literature took, the issues it addressed and the various disciplines and sources on which it drew are analysed; special attention is paid to the relation between this body of diplomatic theory and contemporary diplomatic practice; and, finally, an assessment of the contribution that this literature made to both the renewal of ius gentium and the “professionalisation” of ambassadors is undertaken.