Impact investing integrates financial returns with measurable social and environmental benefits, distinguishing itself from other responsible investment strategies through intentionality, financial return expectations, rigorous impact measurement and management (IMM) and contribution beyond capital, emphasising investor engagement beyond financial support. This chapter examines its foundational principles, structures, financial instruments, and regulatory landscape. It traces the definitional evolution of impact investing, highlighting contributions from key organisations like the Global Impact Investing Network (GIIN) and the Organisation for Economic Co-operation and Development (OECD). Additionally, the chapter reviews investment structures and models to illustrate how capital is strategically allocated to drive positive outcomes while balancing risk and return. The legal and regulatory landscape is also analysed, focusing on jurisdictional differences, disclosure requirements, and investor protection mechanisms to ensure credibility and prevent impact washing. Legislative developments, such as the Sustainable Finance Disclosure Regulation (SFDR) and the UN Principles for Responsible Investment (PRI), are discussed for their role in standardising reporting and enhancing transparency. Despite its growing acceptance, impact investing faces challenges like mission drift, impact washing, and measurement standardisation. Strengthening regulatory mechanisms, ensuring investor accountability, and refining impact measurement frameworks remain critical to sustaining its credibility and effectiveness. By comprehensively exploring these aspects, this chapter positions impact investing as a vital tool for aligning capital with sustainable development objectives and fostering long-term positive change in global markets.