Sustainable finance is a long-term approach to finance and investing, emphasizing long-term thinking, decision-making, and value creation. Impact investing is an emerging category of sustainable finance aimed at creating a positive environmental or social impact through identifying and funding entrepreneurs working to solve environmental or social problems while achieving a financial return. One of the foundational premises of impact investing is that society’s most challenging problems can only be addressed by harnessing entrepreneurship, innovation, and capital. This chapter discusses the core principles of impact investing and the emerging impact investing ecosystem. It also describes the inherent linkages between impact investment and social entrepreneurship. The chapter continues with an overview of impact investment tools and structures, impact measurement and management, and impact reporting, and it closes with thoughts on the future of impact investing.