The world is spending only $3 trillion a year on infrastructure, half of the $6 trillion a year required from 2015 to 2030. Given the scale of investment requirements and limits to public-sector financing capacity, increased private-capital mobilization for long-term infrastructure investment, especially in developing and middle-income countries, will be critical. This report examines how to create the financing environment that could create a step change in private-sector capital flows into sustainable infrastructure. Financing the level of sustainable-infrastructure investment consistent with climate change and Sustainable Development Goals faces three challenges. First, there is not enough private-sector financing for infrastructure in general. Second, even if more private capital is put to work, it will be difficult to ensure that it goes to sustainable infrastructure in particular. Third, a significant share of this financing needs to be redirected toward developing and middle-income countries, where the majority of demand will be concentrated.