This article addresses the relational dynamics of interorganizational relationships where multiple legally independent organizations work on a joint goal, for example in public–private partnerships, alliances, or joint ventures. It focuses on the dynamics of groups that consist of members representing different organizations and thus different interests, who come together to work on the multiparty task. The relational dynamics are understood from a so-called systems-psychodynamic perspective, which aims to understand the emotional life of social systems in context. The article first will depict the relational challenges of working across organizational boundaries. It then will briefly sketch how social psychology (the domain par excellence for studying intergroup relations and group dynamics) helps fathom the relational challenges and where its insights are incomplete. Then, a systems-psychodynamic perspective is introduced. The article proceeds with describing an action research approach that is sensitive to the emotional underpinnings of interorganizational relationships, by providing two illustrations: one involving a real-life infrastructural project, the other concerning a complex behavioral simulation of interorganizational dynamics. The article ends with some reflections on the use of a systems-psychodynamic perspective in understanding and working with multiparty dynamics.