Even without legal ownership, groups can experience objects, places, and ideas as belonging to them ('ours'). This state of mind-collective psychological ownership-is understudied in social psychology, yet it is central to many intergroup conflicts and stewardship behaviour. We discuss our research on the psychological processes and social-psychological implications of collective psychological ownership. We studied territorial ownership, in different parts of the world and at different geographical levels, offering not only a cross-national but also conceptual replication of the processes. Our findings show that collective psychological ownership is inferred based on primo-occupancy, investment, and formation. Further, we demonstrate that collective psychological ownership can have positive intragroup and negative intergroup outcomes, which are guided by perceived group responsibility and exclusive determination right. We then discuss ownership threat (losing what is 'ours'), and we consider the role of group identification in ownership-related processes. We conclude by providing directions for future research.