This paper introduces the ways in which the poorest are defined, conceptualised and counted, and attempts to identify the characteristics that make such poverty intractable in the face of policy. We apply the concepts of absolute and relative poverty, as well as those of poverty depth, breadth and duration, to explore where to draw a line between the ‘merely poor’ and the poorest. We suggest that the poorest tend to be those who face, or have faced, multiple challenges in different spheres of life, and outline the causes of poverty that is absolutely or relatively severe, multidimensional and/or persistent. The paper concludes with a brief discussion of why development policy should be concerned with the poorest at all.