This chapter defines non-governmental organisation (NGO) accountability and self-regulation laying down the conceptual foundation of this book. It defines NGO accountability by answering four key questions: why accountability? For what? To whom? How? It also explores why NGO accountability is problematic and how NGOs are underperforming in this area. Subsequently, it overviews how NGOs are regulated under national and international laws, emphasising the main difficulties of regulating NGOs while promoting their accountability. After that, it defines self-regulation, shedding light on nature, drivers, forms, aims, the institutional design of self-regulatory instruments, and their conditions of effectiveness. Finally, it focuses on the institutional dysfunctions and regulatory concerns that typically affect self-regulation in addressing NGO accountability: difficulties in codifying beneficiary accountability, design flaws in terms of monitoring and sanctioning and the issue of non-compliance.