Peacebuilding is nowadays characterised by its polymorphism, with a variety of ostensibly complementary actors involved in the political, socio-economic and military domains. Instead of focusing on institutional discourses or designs that often prevail in academic literature, this special issue returns to concrete interventions. It addresses mechanisms of inter-organisational cooperation in the making. Based on the study of various local settings (Somalia, Liberia, Burundi and Kosovo), the three articles offer new insights into on-the-ground workings of cooperation. They show a discrepancy between the high-value international actors – first and foremost, international organisations – attach to cooperation and the frequently lacklustre results and occasional reverses thereof. On top of that, this special issue relies on several sociological tools and methods that are helpful to understand the complex interactions involved in inter-organisational cooperation in peace processes.