This Special Issue of Eastern European History Review deals with the history of diplomacy and its executors- diplomats (ambassadors, legates, nuncios, internuncios, agents and others)- in the seventeenth century's Polish - Lithuanian Commonwealth. As a state conglomerate culturally and politically, closely connected to Weastern Europe, and for its specific istitutional form of a "noble republic", the Commonwealth has created, in the field of diplomatic practice, istitutions, principles and ceremonies anique in the European context. The focus of the volume is the histographic stream of the New Diplomatic History, without withdrawing from classic approaches that remain an important field of research. Tracing the salient moments and facts of the complex history of the seventeenth century of the Polish - Lithuanian Commonwealth, the volume - with the participation of authors from different parts of Europe and beyond - constitutes an international opportunity to reflect on issues and topics of old and new diplomatic history. Diplomats and Diplomacy in the Early Modern Polish - Lithuanian Commonwealth (XVII century) constitutes a result of the research activity and scientific collaboration inherent to the project directed by Dr Dorata Gregorowicz The Holy See and the crisis of sovereignty of John II Casimir Vasa and Michael Korybut Wisniowircki's election (1660-1669),n. 2018/28/C/HS3/00176, financed by the National Science Centre, Poland.