This introductory chapter will attempt to frame the concept of “ensuring that no one is left behind” in the context of the 2030 Agenda, and in particular in the context of the sustainable development goals and targets, in a simple fashion. It will examine how specific goals and targets across the SDGs relate to this notion. Based on this, it will highlight key lines of scientific research and debate from various disciplines that are related to “ensuring that no one is left behind” in its various nuances and in relation to the various goals and targets. The chapter will serve as an anchor for other chapters, in particular chapters 3 and 4.

Key questions to be addressed:

• How does “ensuring that no one is left behind” relate to other concepts in the 2030Agenda and to SDG goals and targets?
• What are implications of the principle of leaving no one behind for implementation strategies across different SDG areas?
• What are the main strands of scientific inquiry and evidence covering this question for various areas in the SDGs?