International responses to conflicts and humanitarian emergencies have become more crowded. Not only do traditional actors intervene on a greater scale, such as non-governmental organizations and the military, but new actors such as Private Military Security Companies also play an increasingly important role. These actors often differ in their precise objective and the constituencies they are accountable to. Yet, the practices of these actors in operations are intertwined and many of their tasks overlap. Improved communication and coordination of these actors in complex operations can thus be expected to lead to strong increases in mission effectiveness. This Special Issue provides a conceptual platform to understand and explain under what conditions coordination among these actors occurs. The articles cluster around three themes. A first set focuses on who the actors involved are along a private vs public and military vs civilian divide; a second group of articles assesses experiences of coordination of different actors in the field; finally, a third looks at a plurality of elements that may trigger or hinder coordination.