How do actors undertake institutional design in complex systems? Scholars recognize that many international regimes are becoming increasingly complex. Yet relatively little is known about how actors design or redesign institutions amid this complexity. As participant-observers in the UN negotiations on investment treaty reform, we have watched state officials and other participants grapple with this question for several years. To help explain what we have observed, we conceptualize these participants as complex designers -actors who seek to design and redesign institutions within complex adaptive systems. We then formulate three emergent design principles that seem to guide their approach as they aim to create: flexible structures, balanced content, and adaptive management processes. In a dynamic era marked by unpredictability, division, and complex transnational challenges, we believe these concepts may prove to be increasingly relevant in global governance.