Mobilising capital for a sustainable economy requires action on two fronts. Current capital allocation must be shifted from an unsustainable pathway to a sustainable one and the investment gap must be filled to ensure that sustainable economy objectives are achieved; this includes fulfilment of the Paris Agreement on Climate Change and the UN Sustainable Development Goals. This chapter focuses on the latter issue: filling the investment gap. Primarily, it focuses on the example of tackling the climate change challenge, although these ideas can be applied to the wider environmental and social agendas too. It discusses how to: understand and articulate investment needs; adapt market arrangements using policy to encourage investment; improve the efficacy with which public finance crowds in private finance; and ensure impact through positive real economy outcomes. Throughout the chapter, real-world examples of the 'policy prescriptions' are offered to turn theory into practice.