The global community has spoken loud and clear: more resources must be mobilised to end extreme poverty and mitigate the effects of climate change. Blended finance - an approach to mix different forms of capital in support of development - is emerging as an important solution to help raise resources for the Sustainable Development Goals in developing countries. But scaling up blended finance without a good understanding of its risks could have unintended consequences for development co-operation providers. This report presents a comprehensive assessment of the state and priorities for blended finance as it is being used to support sustainable development in developing countries. It describes concepts and definitions, presents an overview of actors and instruments, and discusses lessons learned from blending approaches, tracking and data, and monitoring and evaluation. Its findings and recommendations are useful for policy makers and practitioners.