This study critically reviews and assesses the large body of evidence on the impacts of agricultural research by the CGIAR and its partners in South Asia. The long history of research, the extensive databases available and the vast literature on impacts that exist in this region provided a fertile ground for this study, which aimed to systematically examine and understand the complexities of how research has led to outputs, uptake, outcomes and impacts, and the distributional consequences of these. Implications and lessons are drawn from this synthesis of the literature to address the issues of the gross (positive and negative) and net payoffs from past investments by the CGIAR and partners (the accountability question), as well as to help shape current and future priorities (the institutional learning question). The study also identifies the knowledge gaps and researchable questions that should improve our understanding of opportunities for, and impediments to, agricultural technology enhancement as a strategy for achieving future CGIAR goals, namely poverty alleviation, food security, and environmental sustainability. Impact Brief No. 21 for this study is titled "Impact of agricultural research in South Asia since the Green Revolution" and is available on the site.