How does institutional complexity affect effectiveness? This chapter addresses this question for the three policy fields studied in this book: renewable energy, fossil fuel subsidy reform, and carbon markets. The chapter takes a comprehensive perspective across the three case studies by examining three dimensions of effectiveness: output (generating regulations and intrastructure), outcome (changing behaviour), and impact (solving the problem). The study relies on a two-track approach, integrating assessments by researchers and interviews with key stakeholders. The results show how the considerable institutional complexity in the climate-energy nexus has consequences for effectiveness. Notwithstanding the methodical challenges for evaluating effectiveness under conditions of institutional complexity, these insights demonstrate that such an assessment is of high importance and should be continued for other contexts of global governance. In particular, the findings of this chapter help to identify suitable management options – i.e. options for formally regulating the linkage between institutions – for the climate-energy nexus. With these suggestions and its conceptual and empirical novelty, the chapter contributes to a variety of literatures – on climate and energy governance, on institutional complexity, and on effectiveness – while being of interest to different stakeholders operating in the climate-energy nexus.