The purpose of this explanatory note is to provide further clarification about the challenges to the development of social impact investment markets and the potential opportunities for action by governments. The note aims to act as a resource for government policy makers considering the development of their own social impact investment market. This note is the result of analysis conducted on the existing and potential ecosystems in the countries of the Social Impact Investment Taskforce. It draws on the findings of the Taskforce’s National Advisory Boards and Working Groups, and a cross‐country expert group provided further guidance and input. Additionally, this note describes the structure of the social impact investment ecosystem and sets out the leadership challenges and the range of policy opportunities that policymakers have to address. The policy levers outlined in this note will need to be considered and applied within each country’s historical and institutional context. Thus, the specific opportunity for each government will differ according to overarching policy priorities and the existing nature of social service provision.