The following introduction reviews the gaps in extant International Relations theory regarding foreign intervention in ethnic and ethnonational conflicts, proposing ways in which Rationalist and Constructivist approaches to International Relations could contribute to our understanding of such phenomena. The introduction then provides an overview of the special issue's five articles, which draw upon evidence from Africa, the Balkans, Northern Ireland, and the Basque Country to highlight the benefits and drawbacks of third-party intervention in ethnic and ethnonational conflicts. In doing so, they highlight many of the most salient issues affecting the practice of intervention and peacebuilding today.