This special issue discusses, in-depth, the embedded conundrum of South-South and triangular cooperation (SSTC) whose frontiers are shifted from collaboration to contention within the United Nations (UN) development system and beyond. This introductory article provides the conceptual framework—the contentioncollaboration spectrum—that guides all the contributors and serves as the collective starting point for this project. The moving frontiers of SSTC reflect the shifting historic relationships between the global South and North as well as Southern partner countries. The framework enables the six articles of this special issue to investigate the paradoxical structure of contrasting dynamics of SSTC, which has always been exposed to historical transformations at multi-levels of analysis: global governance, regional engagements, middle power perspectives, and the UN development system and beyond.